Amirloulou

Actually, the inverter checks its output with the same sense resistors that it uses to figure out how much PWM to give the IGBTs. So that doesn't sound likely after all. It has to be some sort of control issue, which is handled by the firmware. I suppose if the bypass relay is stuck on, it would read utility voltage, which could be over 250 VAC some of the time. So as the service manual suggests, try running with no utility and no load. If no problem for a while, turn on loads; if no problem for another while, turn on the AC input.
Perhaps a particular load might cause the inverter to become unstable; a few such loads have been identified. So if the problem occurs only with the loads on, try turning off all loads and gradually turn them back on to see which one might trigger the issue.