My guess is you have a noise problem, which is made worse by the higher voltage.

I've just had a quick scan of the G540 manual. How have you connected pin 10 and 12 on the G540? Have you tried connecting them with just a short bit wire?

The G540 uses 4 G250s internally, which don't have any form of fault output, so an individual axis fault should not cause the G540 fault light to come on.