what model generator
sounds like you might have the Genirac inverter model
and the 120v AC output voltage is too high
the fault definition for cranking the engine , means low voltage to the running genset when trying to start the drive engine
this should not exist at this point with genset being the only thing running
could be a bad battery
where is the genset connected? engine battery or house/rv batteries
if the converter/charger only charges the house from shore or genset and the genset is connected to a low/bad engine/chassis battery, this might cause your problem
but the inverter too high does not seem too match engine crank battery low