(white screen appears with a single ASCII character in the top left), the solution provided by Sage support was as follows:
1) All users out of MAS
2) Stop the MAS Application that runs on the server
3) On the server, rename the following files (they will get recreated)
a. e:\accounting\mas430\mas90\mas_system\sy_console.m4t
b. e:\accounting\mas430\mas90\mas_system\sy_workstation.m4t
c. e:\accounting\mas430\mas90\home\lib\_appserv\locate.pvk
d. e:\accounting\mas430\mas90\home\lib\_appserv\sessions.pvk
4) If you cannot rename the files, then the file is in use and is most likely causing the corruption
5) Restart the server, if needed
6) When server restarts, you need to make sure that MAS Application is not running
7) Rename the file(s) that you couldn't before the restart
8) From the Start List, select Sage Software | MAS Application Server Configuration (this will recreate the sessions.pvk file), then close this screen
9) From the Start List | Startup, run Application Server Startup
10) Go into MAS 200 Desktop to ensure that you can get into MAS, then close out of MAS
S11) Disconnect (do not log off) from the Server