APV

@charlez - I don't think the suppliers are even aware of this issue with the inverter!
Here is the circuit

The heart of the isolater, is the LD03-16B12 (AC/DC Converter Vin = 90-528VAC (100-745VDC) Vout = 12V 250MA).  Since I have 2 banks of panels consisting of 400W panels capable of delivering more than 20A in total, I had to use a 30A relay.  If you have smaller panels, you can use a smaller relay.  Just make sure that the coil resistance of the relay is more than 70 Ohms to limit the current draw from the AC/DC converter.
Of course you can also use any other type of DC/DC (or AC/DC) converter.  As long as it can handle a relatively high input voltage with an output current of around 250mA at 12V (Or whatever voltage you desire to trigger the relay)
I hope this is useful for everybody who experiences Error 08 while the PV panels are very wet and the inverter is in bypass mode...

@charlez has pointed me to this information from the official Axpert MKS II manual:
14. WARNING: Because this inverter is non-isolated, only three types of PV modules are acceptable: single crystalline, poly crystalline with class A-rated and CIGS modules. To avoid any malfunction, do not connect any PV modules with possible current leakage to the inverter. For example, grounded PV modules will cause current leakage to the inverter. When using CIGS modules, please be sure NO grounding.
This corresponds to my findings that a large percentage of the Error 08 problems are related to the design of the inverter.  In other words, when in bypass mode, the PV input must have no leakage to earth, else the bus voltage just rises until it trips.
So I have constructed an automatic isolator.  It disconnects the PV panels when they deliver less than 70V.  By that time, my inverter has long gone into bypass.  The PV panels are reconnected when they generate more than 15W (and the voltage is higher than 70V).  I call it the Error 08 Buster
I tried to keep it as simple as possible.  It works really well.  I had to use the bulb (microwave oven lamp) to prevent the relay from being activated before the panel can generate enough power (not voltage).  Without the bulb (dummy load), the relay turns on and off every 2 seconds in the morning before sunrise.  
These components cost around R500, which I think is a bargain considering how much frustration this problem has been causing me.  Someone even described it as "worse than load shedding" ...
  Today we had rain, but not too much.  Not sure if anyone else in the Cape Town area experienced the error, or are going to experience it tonight, but I won't.  (I hope!!!!)

 

@charlez - I don't think the suppliers are even aware of this issue with the inverter!
Here is the circuit

The heart of the isolater, is the LD03-16B12 (AC/DC Converter Vin = 90-528VAC (100-745VDC) Vout = 12V 250MA).  Since I have 2 banks of panels consisting of 400W panels capable of delivering more than 20A in total, I had to use a 30A relay.  If you have smaller panels, you can use a smaller relay.  Just make sure that the coil resistance of the relay is more than 70 Ohms to limit the current draw from the AC/DC converter.
Of course you can also use any other type of DC/DC (or AC/DC) converter.  As long as it can handle a relatively high input voltage with an output current of around 250mA at 12V (Or whatever voltage you desire to trigger the relay)
I hope this is useful for everybody who experiences Error 08 while the PV panels are very wet and the inverter is in bypass mode...

Yes, your statement is 100% correct, IF you interrupt the DC from the PV panels under full load.
However, in this scenario, the PV panels are disconnected when they are delivering very low power...
Nice demonstration though

I do not think a Sonoff switch will work, since it only breaks one of the 2 wires.  You will still have leakage to earth via the other (unbroken) PV wire.  I really think you need to find a device that has a double pole relay built into it so that you can break both wires coming from the solar panels.
Alternatively, you can use the Sonoff to control a double pole relay with a 240V coil.  That setup will isolate the PV panels completely as well.

If the problem disappears when you disconnect your panels, you need to build an auto-isolator like I designed and posted - that was the only solution that worked for me. It is only applicable to the following situation:
<SPAN STYLE='background:dddd00;'> 1. Inverter in bypass mode AND 2. Solar panels are wet. </SPAN> Regards
A

Firstly, are you using the model with the very high PV input (up to 450V)? The circuit will only work for these models with PV panels attached.
As @Coulomb has mentioned (more than once), the problem can also occur when the battery voltage is too high.  Have you taken any measurements when you ran into error 08?

Hallo @Classna
You can find the relay at Aliexpress at https://www.aliexpress.com/item/32835375697.html?spm=a2g0o.productlist.0.0.e8d73424MccJGE&algo_pvid=7aedc1e2-263b-4d06-af6e-a4b34421831d&algo_expid=7aedc1e2-263b-4d06-af6e-a4b34421831d-23&btsid=0be3743615958382138547685e0790&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_
I cannot find the AC-DC converter at Aliexpress though  As @charlez has mentioned, it is widely available, e.g. https://www.edgeelectronics.com/ac-dc-converters/ld03-16b12/
I am also uploading the schematic again - the capacitor values were displayed incorrectly!
 

Hallo @charlez
Why 40A?  A 335W panel has a short circuit current of less than 10A.  With 2 strings, you need less than 20A.  I am using 405W panels (7x2) and used a 30A relay. 
You can get a 30A from Communica  - https://www.communica.co.za/products/jqx30f-2z-dc12v-flange
I also got the DC-DC converter from them
JQX30F-2Z-DC12V FLANGE
Relay • Flange • Form 2C • VCoil= 12V DC • IMax Switching= 30A • RCoil= 96Ω • 6.3mm Terminal SKU: JQX30F-2Z-DC12V FLANGE
BRAND: Asiaon
 
IN STOCK
R 152.38 (inc VAT)

@charlez - I don't think the suppliers are even aware of this issue with the inverter!
Here is the circuit

The heart of the isolater, is the LD03-16B12 (AC/DC Converter Vin = 90-528VAC (100-745VDC) Vout = 12V 250MA).  Since I have 2 banks of panels consisting of 400W panels capable of delivering more than 20A in total, I had to use a 30A relay.  If you have smaller panels, you can use a smaller relay.  Just make sure that the coil resistance of the relay is more than 70 Ohms to limit the current draw from the AC/DC converter.
Of course you can also use any other type of DC/DC (or AC/DC) converter.  As long as it can handle a relatively high input voltage with an output current of around 250mA at 12V (Or whatever voltage you desire to trigger the relay)
I hope this is useful for everybody who experiences Error 08 while the PV panels are very wet and the inverter is in bypass mode...

@charlez - I don't think the suppliers are even aware of this issue with the inverter!
Here is the circuit

The heart of the isolater, is the LD03-16B12 (AC/DC Converter Vin = 90-528VAC (100-745VDC) Vout = 12V 250MA).  Since I have 2 banks of panels consisting of 400W panels capable of delivering more than 20A in total, I had to use a 30A relay.  If you have smaller panels, you can use a smaller relay.  Just make sure that the coil resistance of the relay is more than 70 Ohms to limit the current draw from the AC/DC converter.
Of course you can also use any other type of DC/DC (or AC/DC) converter.  As long as it can handle a relatively high input voltage with an output current of around 250mA at 12V (Or whatever voltage you desire to trigger the relay)
I hope this is useful for everybody who experiences Error 08 while the PV panels are very wet and the inverter is in bypass mode...

@charlez - I don't think the suppliers are even aware of this issue with the inverter!
Here is the circuit

The heart of the isolater, is the LD03-16B12 (AC/DC Converter Vin = 90-528VAC (100-745VDC) Vout = 12V 250MA).  Since I have 2 banks of panels consisting of 400W panels capable of delivering more than 20A in total, I had to use a 30A relay.  If you have smaller panels, you can use a smaller relay.  Just make sure that the coil resistance of the relay is more than 70 Ohms to limit the current draw from the AC/DC converter.
Of course you can also use any other type of DC/DC (or AC/DC) converter.  As long as it can handle a relatively high input voltage with an output current of around 250mA at 12V (Or whatever voltage you desire to trigger the relay)
I hope this is useful for everybody who experiences Error 08 while the PV panels are very wet and the inverter is in bypass mode...

@charlez has pointed me to this information from the official Axpert MKS II manual:
14. WARNING: Because this inverter is non-isolated, only three types of PV modules are acceptable: single crystalline, poly crystalline with class A-rated and CIGS modules. To avoid any malfunction, do not connect any PV modules with possible current leakage to the inverter. For example, grounded PV modules will cause current leakage to the inverter. When using CIGS modules, please be sure NO grounding.
This corresponds to my findings that a large percentage of the Error 08 problems are related to the design of the inverter.  In other words, when in bypass mode, the PV input must have no leakage to earth, else the bus voltage just rises until it trips.
So I have constructed an automatic isolator.  It disconnects the PV panels when they deliver less than 70V.  By that time, my inverter has long gone into bypass.  The PV panels are reconnected when they generate more than 15W (and the voltage is higher than 70V).  I call it the Error 08 Buster
I tried to keep it as simple as possible.  It works really well.  I had to use the bulb (microwave oven lamp) to prevent the relay from being activated before the panel can generate enough power (not voltage).  Without the bulb (dummy load), the relay turns on and off every 2 seconds in the morning before sunrise.  
These components cost around R500, which I think is a bargain considering how much frustration this problem has been causing me.  Someone even described it as "worse than load shedding" ...
  Today we had rain, but not too much.  Not sure if anyone else in the Cape Town area experienced the error, or are going to experience it tonight, but I won't.  (I hope!!!!)

 

@charlez has pointed me to this information from the official Axpert MKS II manual:
14. WARNING: Because this inverter is non-isolated, only three types of PV modules are acceptable: single crystalline, poly crystalline with class A-rated and CIGS modules. To avoid any malfunction, do not connect any PV modules with possible current leakage to the inverter. For example, grounded PV modules will cause current leakage to the inverter. When using CIGS modules, please be sure NO grounding.
This corresponds to my findings that a large percentage of the Error 08 problems are related to the design of the inverter.  In other words, when in bypass mode, the PV input must have no leakage to earth, else the bus voltage just rises until it trips.
So I have constructed an automatic isolator.  It disconnects the PV panels when they deliver less than 70V.  By that time, my inverter has long gone into bypass.  The PV panels are reconnected when they generate more than 15W (and the voltage is higher than 70V).  I call it the Error 08 Buster
I tried to keep it as simple as possible.  It works really well.  I had to use the bulb (microwave oven lamp) to prevent the relay from being activated before the panel can generate enough power (not voltage).  Without the bulb (dummy load), the relay turns on and off every 2 seconds in the morning before sunrise.  
These components cost around R500, which I think is a bargain considering how much frustration this problem has been causing me.  Someone even described it as "worse than load shedding" ...
  Today we had rain, but not too much.  Not sure if anyone else in the Cape Town area experienced the error, or are going to experience it tonight, but I won't.  (I hope!!!!)

 

That is bad news @charlez :(  If (and only if) moisture is part of the problem, it could be that the cables were not completely dry after laying on the roof. So maybe there still was some leakage in your case... I have to tie mine up like you did in any case, since rain causes my earth leakage to trip.  So my system is even worse than yours :)

@charlez Yes, it goes without saying   I guess I should have mentioned it for completeness sake, but from the beginning of my contribution towards this thread, I have stated that my problems only occur when the unit is in bypass mode...
 

Today it was raining and of course my Error 08 appeared.
First I measured the leakage current into the PV panels.  About 3mA AC per bank (I have 2 banks).
Then I switched off the panels and started logging.  My samples are about 3s apart.
As you can see from the graph, the bus voltage is stable around 372V with the panels off.
At 19:45:04 I switch on the panels and the bus voltage IMMEDIATELY starts to rise.
At 19:45:44, it reaches 465V.  Then I switch off bank 1 of the panels .  The voltage almost stabilises at that point.
At 19:46:01, I swap the 2 banks.  So now bank 1 is active and bank 2 is off.  The voltage continues to rise, but fairly slowly.
At 19:46:20, I switch off both banks again and the bus voltage starts to drop until around 19:48:20 when it stabilises at 372V again.
 
In the meantime, the utility voltage rises and falls a bit, but I find no correlation between the utility voltage and the bus voltage.  All the other parameters are fairly constant as well
 
So if this does not show that the problem is closely related to leakage from the PV panels, I don't know what to think

Unfortunately the PV input of the Axpert is not isolated as @Coulomb pointed out.  So this means any leakage to earth on the panels, whether through connectors or the panels itself, can be a contributing factor towards our problems.  However as @JustinSchoeman pointed out, it definitely is not the only factor.  He uses the Axpert as a UPS only, so there are no panels involved!!!  I hope his setup is the exception to the rule.  The earth leakage issue does seem to be very relevant in the other cases, since you ( @charlez ) have also managed to prevent further occurrences of Error 08 by simply isolating the panels (after the first occurrence of the error).

OK, so now there are 3 of us with the same type of installation and the same issue - I smell a rat
 

Hallo @charlez I believe that simply lifting them from the surface of the roof with cable ties (or whatever) should be fine.   It will be quite a mission for me to modify my wiring, since I have grouped the panes in a chunk of 8x2 with only a 1cm gap between them.  Hopefully I can reach the cables from the lower and upper sides of the array...
 My problems only occur in fairly heavy rains.  I use the CANADIAN SOLAR 405W SUPER HIGH POWER POLY PERC HIKU panels with MC4 connectors, so it is the same brand...

This morning got Error 08 again after some rain. Thanks to @APV by isolating the PV array I was able to restart the inverter without it crashing again. With the PV array connected the bus voltage would have spike again requiring multiple starts. My earth leakage did not trip.

When the panels are disconnected in this state, there is a voltage of about 50V at the panel connector on the inverter.  I am going to insert a diode between the panels and the inverter and see if it makes any difference...
 

Here is something extremely interesting!!!
The inverter currently is in its bad mood.  When you switch it off at the bottom, the bus voltage goes to zero.  When you switch it on again, the voltage quickly goes to 380V and then rises quite fast until it trips.  Out of sheer curiosity / desperation, I switched off the solar panels.  The bus voltage dropped to 380V in a matter of 30s!!!  Now remember - the unit is in bypass.  It is pitch dark outside.  I have measured the solar panel input - just about 0V.  So why the heck would removing the panels, "solve" the problem???

Great work BvR!  I don't know how you managed to communicate with Voltronic, but I am very impressed!  So now my inverter is running 7.80.  We now have to wait for a cloudy day to see if the issue is resolved....
 

Well, as I have said, the problem only occurs at night.  No solar input. causing the issue.
However, to answer your question.  I have 2 banks of 7x400W panels in series, connected in parallel.  Thus giving me maximum open circuit voltage of about 350V DC and a maximum current of 20A, at least theoretically.  Also a theoretical power of 5600W.  For these cloudy days, you know I am aware that it exceeds the maximum solar input of the Axpert (4500W), but the Axpert only uses up to 4500W.  I have tested it...