Hello Kevin,
yes, i´m using an inverter generator - with the following scheme:
self-excited induction generator → chokes → 3ph thyristor rectifier → dc-link → 3ph igbt-inverter - lc-filter - dyn5 isolation transformer.
I think it is ESS related. If i guess right (but I could also be completely wrong not knowing the exact inner workings of that ESS gird-parallel thing), without ESS the Quattros are behaving more passive - they use what they get on the input, step it down, rectify and charge. With ESS i presume they are using a grid-parallel active method, where they create there own sine-wave synchronous to the input which they modify in amplitude and phase to control the direction of flow of active and reactive power. If my assumptions are right, then the power quality requirements would be much higher in ESS mode because every mismatch in waveform shape would result in equalization currents between the quattros and the genset.
If it comes from a bad waveform than i have suggestion for a test that might sound completely crazy but could actually work. I had once a case that the servers etc. in our company had to be supplied for serveral hours by generator due to transformer maintenance and the UPSs didnt take the power because the waveform was too bad. Load was approx. 5kW and generator was 16kVA, synchronous, AVR. No chance. And it had to - whatever necessary. 
Now the crazy mac-gyver solution part - i took a 5kW (or 7,5kW?) 3ph induction motor, compensated it with three capacitors in delta to around cos-phi 0,98-0,99 and connected it parallel to the genset. Free-running, whithout load. This contraption forced and ironed out the distorded waveform into shape “good enought” for the UPSs to now accept the generator power. And it ran reliably the whole day 
I’m looking forward to your test results and whish you luck. 