Today, I received a new main board for the inverter supplied "for free" by EASun. For free means that I should pay $30 for shipping but the shipping was $39.92 + VAT and VAT handling fees ending up in $78.52...
Changing the main board did absolutely nothing because, as far as I can see, the CPU is on a daughterboard plugged into the main board and I didn't get a new one so back to square one... As long as it was a firmware upgrade that was the cause of the inverter to die, it is only logical that it is the CPU board that needed replacing.