If you have MAS90 Account System and you get an error that the Session Tracking File is corrupt, get everyone to log out of the MAS90 server.
Now get on the MAS90 server at your office, look at this path:
C:\Program Files\Best\MAS90\home\session
Move out all the files out of there and stick them in:
C:\Program Files\Best\MAS90\home\oldsession
You may have to create the "oldsession" folder under "home".
That leaves the "session" folder empty.
Now have everyone get in again.
Problem solved. Hope this saves you the money of an expensive support call for a bug in the software that you shouldn't have to pay for in the first place.