Citrix sessions launch then immediately log out.

I have a set of 8 server 2008 servers running Xenapp 5.0 and some users are having issues launching apps.  This started happening only recently and I can't tie it to any changes on the servers or the user's PCs.  A user will launch a published app, the app loads, the app UI pops up on the screen for maybe 1 second, then the session logs out.  This is the case for a large number of users on a wide number of system.  Some users are not experiencing this issue though they are connecting to the same apps on the same servers. For example, sessions launching from my PC (regardless of which ID is used) work every time. Other apps on other 2008 servers in the same farm work as well on the machines that have issues with the set of 8 servers mentioned above.  I don't see any unusual messages in the event log. I have tried to implement the LogoffCheckerStartupDelayInSeconds fix with no luck (though perhaps that won't work for XA5?)

Any help would be appreciated.