We had a user call our help desk stating that they weren't able to log into their XenApp 6.0 published desktop. It accepted their credentials, started going through the motions of applying group policy, etc., but before the desktop actually appeared, the session disappeared completely. After doing some testing, I noticed that an error message was popping up, for less than a second, right before the session closed. With my trigger finger, I was able to snap a screen shot to discover the following error: "A device attached to the system is not functioning."
After poking around the Citrix Delivery Services Console, I discovered the that the user had a Disconnected session out there, that was obviously in a completely cheddared state. So, when the user was logging in, they were being reconnected to their broken session, and were never able to log in. After logging off the disconnected session, the user was able to start a brand new session and log in successfully.