ArnoT

This is not 100% correct. The Voc at 25°C is based on a irradiation of 1000. Early morning the irradiation could be 200 or any other value below 1000. This in turn would cause the actual Voc to be say 25V instead of 50V per panel for the panels under discussion. This for even 10 panels would be 250V. This far exceeds the start up voltage of the MPPT and as even the lowest current is produced would drop the Voc even further.
I am not saying one should not use the Voc at STC but the risk is much lower due to the low Voc under low irradiation conditions. 
The over voltage can still come in at other possible alignment conditions as per @GreenFields.
This point that the Voc is much lower when at below 1000 irradiation has been explained in earlier topics. 
Currently in Gauteng my Voc=48V panels at 06h30 would only have a 7V/panel. At about 06h45 this will go up to a level that switches on the MPPT on my Axpert and the voltage would drop to 35V/panel under a very low Watt load. Thus the panel never got close to the STC Voc level due to low irradiation. 
I am sure those members with irradiation meters will be able to confirm this behavior by measuring the Voc. 
I stand to be corrected as the above is based on actual values. 
Actual values can even differ with a high % and not get us close to the risky level for this stated condition. 

One could add, there's a +/-3% tolerance on the Isc and Voc values for that panel. Also, the Isc and Voc values are based on STC, but there's also a chance it could be increased at higher levels of irradiance during cloud edge. Basically you'd be skating on thin ice, and you might be okay for a long time, until one day maybe the heavens align against you, and you exceed safe values. If you're interested in 10 panels per MPPT, maybe rather consider 2 strings of 5 in parallel, or the simplest would be to just stick to 8 in series per MPPT.
On a lighter note, tomorrow is the solstice, just hang in there, there's brighter days to come.

This is really disappointing, that would mean that I won't be able to introduce more batteries in the near future. Ferdi from Magneto was at my house and installed/upgraded the firmware on all three batteries to no avail. 
My installer informed me that Magento still have a version 1 in stock and that they will exchange the version 2 with this one and then my problem will be sorted out. This means that I won't be able to introduce a new battery in the near future as there won't be any version 1's left in the market which is totally unacceptable.
My installer also assured me that it won't be a problem to introduce new batteries over the course of a few months and now it is a problem.