Okay - that´s intresting. And crazy.
I´ve set my AC1 Input to the german VDE grid-code with LOM disabled and it works fine so far. I´ve limited the charge current to 50A in VEConfig. Worked without limit as well - it´s only there at the moment because my voltage dropped slightly below 220V at full power. (Different story - homemade capacitor excited induction generator, working on a automated capacitive reactance compensation with static switches by now that will solve that 
)
If i disconnect the grid on AC2 an connect AC1 to the generator, then it takes around 30-60 seconds for the Quattors to sync and close relays. Then they slowly start to ramp up the power - it takes another good minute to reach the desired power draw / charge current. I have three analog amp-meters on the generator - if you look closely, you can see small oscillations repeating approx. every 1-1,5 seconds. But very small - 0,5-1 amp i would say. Engine sounds smooth and on the GX display there arent any fluctuations worth of note.
I have a 64kWh battery an have done hour-long charging runs without problems.
My generator, due to the aforementioned inverter, puts out a perfect sine-wave. Your quattors seem to have some kind of problems regarding power quality. But i´m confused - you have a large and professional generator that should have absolutely no problems delivering clean-enough power.
An idea for further narrowing down the possible cause:
“Simulate” a perfect generator by hooking up the grid on AC1 configured as generator input on the GX and with VDE grid-code without LOM (be careful). Possibly with reduced charge power depending on your supply. If possible attach a clamp meter on one of the phases to look for fluctuations. If this runs and charges as expected, then it has to be a power qualiy issue. I did that for a short test before i connected my real generator the first time to be sure that my config worked in principle and if later there are issues that they then have to come from the Generator.
If Above worked, next thing i would try is hooking up an oscilloscope (with suitable differential probes) to the generator power and have a look on it with and without the quattros attachted and trying to charge. It has to be something, distortion, harmonics that build up or whatever.
Here my generator/inverter waveform (L1/N) when charging with 8kW. (Ignore the 144V - probe setting was wrong).
