That sounds like a file permission issue for the user that is trying to start the session. I would check to be sure that file permissions are set up properly, group membership is set correctly, and would test for a few other users.
It also is important to include a bit more information in posts such as this so that others can help you. I.e. where did you get this error? Was it in the browser? On the server? Is the rstudio service running / listening? Was this after authentication? What version of RStudio Server are you running? etc.