Can anyone please help!!

A friend of mine has a XYZ 560 Mini Mill. The machine shows the following fault message: -

Profibus-DP Fault on drive Y
Code 504 value 0
Acm No 380500

The fault is intermittent and we cannot find the fault. The motor, encoder & the connections between them have been checked. The drive for that axis has been swapped i.e. X-axis drive has been swapped with Y-axis drive but the fault still occurs on the same axis.

Has anybody suffered with the same problem?
Does anybody know what the cause is?