I was working in my demo/training environment today, which is a Project Server 2010 setup, when I encountered this issue. Turned out to be a corrupt user profile in the Remote Desktop Services machine. I logged on as a different user to the machine, deleted the profile in question, then logged back on the machine with the original account. When I logged on i noticed the familiar messages that typically accompany a user logging onto a machine for the first time. This time when I launched Project Professional it logged me in without an issue.