If yes to the above then read on, if not sorry I can't help We've seen this on restarting the server hosting a Windchill 9.1 M050 install,
What we have to do is to shutdown all Windchill services prior to restarting the system and run a script to rebuild certain files.
Here is what we need to do to fix it, this was given to us by PTC
1, stop all services - Windchill, Apache, Tomcat, Windchill DS 2, delete the folder "File" from C:\PTC\Windchill_9.1\Windchill\tasks\codebase\com\infoengine\compiledTasks 3, Delete the ptcserlet folder from C:\tmp 4, delete Tomcat Cache 5, Open a Windchill shell and cd to codebase 6, enter "ant -f makejar.xml" 7, Follow this sequence when you start your server: - WindchillDS>>Apache>>Tomcat>> Windchill
Hope this helps fix your problem,
There was some other users on the exploder that have the same issue and I think there is a SPR with PTC