If I remember correctly this has to do with a corrupt user profile in MAS90 and the work around was to create a new user code for the person/workstation. 

Sounds like the good, old "gray screen with help" issue, but that was only MAS 200 [edit: autocorrect, I guess] (Sage 100 Advanced). That has seemingly morphed into KB # (sorry, "Solution ID:  222924450019199").

BL, I've always recommended renaming and recreating those 4 system files, but as an alternative we've created a new user in Sage to bypass this.

Thank you everyone for chiming in :) - our developer emailed me and said 

1. From a workstation that IS working, open DFDM, and then open the SY_User table in the MAS_SYSTEM folder.  Identify the 10-digit UserKey$ for the user having the issue.
2. Now, open the SY_Workstation table, and navigate to the WorkstationName + UserKey$ key combination.
3. Delete this record from SY_Workstation and then close DDFM.
4. Have that user attempt to log into that workstation again. 

And this solved it in case anyone else wants to know.

-Larry