I have the exact same issue -- just 1 user that cannot access the ServiceDesk. Initially, I copied all of his user settings into a temp account, and found that his homepage was set to a page he no longer had access to. I changed this in the temp account and I could log in again perfectly. However, changing this in the user's account did nothing. It has been a couple of months now, with several patches and reboots to the server. The only thing that works is creating a new PM login for that user. (Obviously not tied to his AD login anymore...)