Clement

Error 90 (fault code 90) results from a mismatch between what is in the bootloader and what is in the rest of the firmware. I believe it was designed to catch out clones who copy the firmware but not the bootloader.
You might be able to update to factory firmware version 72.70, which doesn't have the error 90 check, or patched firmware version 72.70c or 73.00e. But first check which model yours are a clone of, or you could brick the inverter. See Can I update my firmware, and to what? and Do I own a clone? . Not all models can be update in this manner.
Unfortunately, some clones aren't able to be updated at all. If that's the case, you need to get a new control board (it plugs in to the main board) for each machine from your supplier, with new firmware that solves this problem. I assume that most clone manufacturers will have sorted this out by now. Don't bother attempting to source control boards from a legitimate importer; they will ask for a serial number, and specifically won't help anyone with a clone.
It's an ugly war between the original manufacturer (Voltronic Power), and the clone makers, and unfortunately people like you are getting caught in the crossfire.
[ Edit: 72.70e → 72.70c; there is no 72.70e. Sigh. ]

Yes. @Chris Hobson called me the other day about that. He had a Goodwe powering loads even though the grid was down. Keep in mind though that the Goodwe has an input and an output, and this was on the output, so no feedback rules are being broken here.
I am also aware that some SMA models have a secure power feature that can power some loads when the power is out. It's meant to power smaller loads.

The recommendation is that you use batteries that are of the same size, manufacture, age and chemistry. Due to practical considerations sometimes one ends up mixing batteries of different ages (replacing a dud battery for instance) or mixing batteries of different capacity 232Ah and 236Ah from the same manufacturer (the difference is essentially minimal).
I would however never mix batteries from different manufacturers or different chemistries.

It is usually not recommended to mix batteries of different chemistries.

grid tie is fantastic but off grid makes it more complex
I once bred a batch of bass and  a light just above  the water would attract insects  which a lot of would land up in the water . the fish were there having a feeding frenzy .
a bug zapper was also added  there and the fish loved the cooked food as well
point is it saved a lot on feed costs
cheers
 

The fish only read during the day
The client uses 100kWh a day requiring 20kWp to be "virtually off grid" - if you want a system to reduce the bill a bit then just about anything will do that to some degree. If the grid is reliable, the fish not too fussy and authorities not too difficult then a grid tie system of whatever size you like and gennie backup is the way to go.

You will need to investigate the grid-tie conditions in your area before heading down this road.
In Joburg where I live you will get 42c for feed in, and charged Time Of Use rates for consumption. Based on the 8hr/14hr split, of your 14hr consumption period you would be charged 302c for 3hour, 125c for 3 hr, and 90c for 8 hr.
[302 x 3]+[125 x 3]+[90 x 8] = 2000/14 =143c average (we can do this because of the constant load, domestic setup is MUCH more complicated calculation).
The ratio of feed in to consumption charge is 143/42 = 3.4
In order to break even on your utility bill under these conditions you will need to produce the following; [4.1 x 8 = 33kWh] + [4.1 x 14 x 3.4 = 195kWh] = 227kWh per day as apposed to the 100kWh per day in the initial proposal.
Now you need 45kWp (R375k) and 3x InfiniSolar 3p 10kW in parallel (R150k) 25kWh battery to give you about 3hr backup (R60k) 300m2 structure required for your 150 panels Installation sundries thumb-suck (R60k assuming 300m2 structure mentioned above is already available) That is a budget of R645k, or you could go with 1st proposal (using something other than Axpert of course) and invest the extra R200k to pay for your replacement batteries for the rest of time. 
 

I'd be okay with using an infini, yes.
It feels all wrong to me to put 6 Axperts in parallel (they are around 8k a pop, so that's almost 50k worth of inverter) when you can buy other inverters -- better inverters -- for the same price. You could put in two Goodwe hybrids, a large infini, or even a 5kva Multi (admittedly that will cost more, external MPPTs) for not much more.
A 5kva Fronius is like 30k... I know it has no backup, but if the grid is dependable and line rental isn't too insane... ?

I know the client wants to go completely off the grid, but I think you should do the math for a grid-tied system as well. Those generally cost way less and repay a lot faster.
One 5kva Fronius and a slightly oversized array. You could probably offset your daytime costs at under 100k ZAR (ie a fifth of the cost) and possibly still cut 30% off the bill. Thumbsuck. If the meter is an old style disk meter that can spin backward (evil evil... I know)...

4.1kW x 24hr = 98.4kWh per day (ouch!), call it 100kWh so you will need about 20kWp array. So a real cheapo solution would be as follows;
6x Axpert MKS 5k in parallel (for the panel capacity, not for the output power which will be way overkill), about R60k Each Axpert with 10x 315Wp panels = 3.15kWp x6 = 18.9kWp (not quite 20kWp but close enough), about R150k Assume 8hr solar day and 14hr solar night, so your battery bank must carry you for about 14hr's on average. 4.1kW x 14h = 57.4kWh supplied by battery, if you use lead acid and limit it to 50% drain you will need 120kWh battery bank. 120kWh / 48v = 2500Ah battery bank, Ritar make a 2v 1500Ah battery for R4000 - you will need 48 of these for a 3000Ah bank about R192k That's about R402k, plus R48k for installation sundries and R50k for me that's about R500k - sounds like a bargain!
Note that this wouldn't be Off-Grid but Grid-Assisted (not grid-tie, you wont be feeding any power back anywhere) so that the Axperts can take power from the grid when needed. You will get probably 80% of your power requirement from the PV system with the grid helping out with the other 20% on them dark and dingy days.
If it was me I would want to save R10k a month on the utility bill to make capex worthwhile - I reckon you will save 2500kWh per month so your power cost must be R4/kWh or higher to give this system the green light - if not then just pay the utility bill.
Sounds like a blast, have fun
 

The beauty of that system is nothing is really wasted. Should outages become sufficiently irritating and/or frequent, you can add the Multi and some batteries later. You'll pay a little extra, but that's more than compensated for by buying extra time, something that is way underrated, money spent tomorrow is always worth less than money spent today, if it has to be spent at all!

Just to clarify, that doesn't mean the multi WILL carry the whole load. The Multi will pretty much carry only the shortfall, if more power is required than the GTI makes. If there is surplus power, the Multi will charge the batteries. If the batteries are full, the Multi will use GFPR to back off the GTI. The reason they Multi needs to be big enough to carry the whole load is because such a setup isn't 100% stable. The GTIs are ultra sensitive to conditions that might indicate an islanding condition, and large changes in load will sufficiently affect both a battery inverter and a diesel generator so that GTIs will disconnect and reconnect again after a minute.
It goes without saying that if you go with a generator instead, it would also h ave to be sized for the full load.

Okay, I have feedback. The Multi will have to be big enough to carry the whole load, otherwise it won't work properly.

I would use a normal auto-start generator.
When grid fails and grid-tie inverter shuts off, then auto-start generator starts and runs loads as desired. When grid returns then generator will auto-stop after settlement period.

Ifr no batteries are needed then Look at SMA  
http://www.smainverted.com/what-is-a-photovoltaic-diesel-hybrid-system/

I was wondering whether we could get a similar video just by installing CCTV in TTT's workshop? 

It is cheaper to buy an extra solar panel and achieve the same result, without the mechanical complexity IMHO.

Doesn't the Axpert have a built-in charge controller?
If you want to add an external one, then due to the way the MPPT works (buck converter) you can pretty much take the peak power of the PV modules and divide it by 48.
12 * 250 = 3kwp, 3000/48 is going to be slightly more than 60A (50 * 60 = 3000). Because you rarely get the peak, 60A will do.
I can tell you what not to buy. Don't buy the WRND controller.
The two controllers I know and recommend are either the Victron 150/70 (70 ampere) or the Microcare 60A unit.
In practice you will be better off with two smaller controllers, eg Victron 150/35.
There is no cheap way to do it, it's going to set you back at least 10k :-/
Batteries are around 300Ah, so recommended charge rate is between 30A and 45A. 60A is 20%, pushing it a bit but okay :-)

Up until a moment before you start measuring your inverter is in line mode and your charging rate is 10A. From the voltage it appears you are in bulk or absorb and your batteries are not fully charged. When you Axpert goes into float (i.e. the batteries are 99% charged) the middle LED does not flash but is on permanently. If it is charging and is in bulk or absorb the LED flashes. Good batteries will still steadily accept 1% of charge when they are full.
You then switch to battery and immediately your voltage drops from 56.8V to 51.7V  2 minutes later. This is purely because you are no longer charging the batteries. 10 minutes later your batteries are at 50.1V which is closer to your batteries true voltage and a little while later you kick back to grid. I suspect your settings for program 12 & 13 are too close together. 
MY grid power has just gone off and in 4 minutes my batteries have dropped from 54.4V to 51.1V with a 12A draw - entirely normal.
I am away tomorrow but post a screenshot of your settings and the Axpert mechanics will sort it out I am sure.
Addition: Now ½ hour later the battery voltage has recovered to 51.4V with a draw of 5A.

.....and then you going distribute to your mates  and forumites. I can see the data on my setup and can download it as a pdf but cannot convert it to something useful (A csv file would have been good).