I have the same issue, 2 X MGX5048s and they would not parallel from new, Changed the Parallel cards, the LCD Panels and still get F81.. I must say I dont think I will be getting anymore MPP Solar products or recommending to any of my customers.
I manage mine with sonoff or Shelly modules... You can have it trigger and have the genset run for up to an hour and then switch off and limit it to certain times of the day etc..