From some of my old notes on alarm 401 this is a servo alarm I think.
Some of the things that could be going on is the incoming voltage or transformer tap settings.
My note also state that the problem could be on the Velocity Control Circuit side of the drives. Also have a note that states that the incoming voltage at the drive could be at it max or below its minumum.
Also you may try checking backlash of the trust bearings of the axis that the 401 alarm is on.
Like I stated my notes are old , so not sure if any of this will be of help to you since alarm code may be different than I am using.
Contact Fanuc USA . They do have some tech there that know the older controls .