Upstation GXT not taking load from generator power

[DanielUK]

We have a small office with an online Liebert Upstation GXT2000VA UPS which has been running fine for about 6 years, with a full battery replacement last year.

We bought a generator, this one to be precise:

http://www.justgenerators.co.uk/pages/Briggs_Pro_Max_6000EA.htm

which has Automatic Voltage Regulation.

We've had a couple of power outages where we've had to use the generator hooked up to the UPS running at its normal load -no problem, ran fine both times and accepted the genarator power as an acceptable AC input.

Yesterday, however, we lost power and the UPS wouldn't accept the generator power, it went straight to battery mode.

We eventually got it to accept the generator power by unplugging all devices then reattaching them one by by one.

What came to light was the fact that it would only allow a certain number of devices to be attached before it would trip and go into battery mode. For some reason, it seems to work with the generator power up to a point but knowhere near the number of devices that we've had plugged into it before. The UPS itself isn't being overloaded, or the genarator for that matter, all is the same as it's ever been.

What should I be looking at, the UPS or the generator?

Thanks

Dan