The power PCB is likely the board that interfaces to the IGBT's and current sensors.
The UD90A is the control board.
If you have that fault one of the two are bad. Replace the drive.
From CT's App Notes:
HF84 Current Offset Trim Failure
This trip means there is a problem with the current feedback on the Drive.
On Unidrive Sizes 1 to 4, the trip is probably due to the power PCB in the Drive but a problem with the UD90A control PCB could also cause this trip. On a Unidrive Size 5 the trip is cause by either UD99 PCB or the UD90A PCB. The interconnects between the PCBs could also cause this trip and should be checked.