Your Generator is probably producing a Modified Sine Waveform rather than a pure 60Hz sine waveform power. Your UPS probably doesn't like all the additional power harmonics. Some devices such as computers, digital TVs, Hi-Fi's etc just don't like modified sine wave power although it sounds like the computer is able to handle the output from the generator although the computer PSU may eventually object as it overheats rejecting the spurious power harmonics. Some devices such as Kill-A-Watt meters really object (start to smolder) when connected to Modified Sine wave power.