zennomind

Thanks for the last three messages/thoughts. I had not been notified that you'd posted.
@P1000 I could have bought a Solis hybrid for a good price but the Axpert IV is still working and so I decided to remove a panel from the string.
I left the system off until a few days ago and just used my second set of panels, inverter and battery bank. I got up onto the roof and removed the last panel from the string. So that drops the Voc from 455 to 417. So correcting for ambient temperature changes would put this string at just below 500V at around -21C. We only drop to around -5C on a few days a year and that's generally early morning temperatures and so this should eliminate any over voltage. The string output is also more healthy now.

It has a little bit of margin, but usually the protection consists of a MOV or tranzorb that will hopefully be able to blow your external PV fuse. Please do not rely on it, because it is not fail-safe. The open circuit voltage of your panels depend on temperature(lower means higher voltage) and insolation (which could be affected by cloud edge effect etc). It is very possible that it was always marginal and that this is the first time that the same weather/atmospheric and load conditions happened at the same time.
The bottom line is that you should lower your string voltage or replace the inverter with one that can handle the higher voltage (or wait until it blows and you have to replace it...)