I'm having a similar problem with the Mid Tier 7.0 with a different webserver. One thing we've discovered is that if someone is currently logged into the client that they cannot log into the Mid Tier. If they shut down all the clients they have open then they can login fine.
Since getting Techs to log out before they go off into the field to fix things is vitually impossible, we're currently trying to figure out a way to remotely kill their clients, or kill their session in Remedy (as one can do manually through the ADmin tool with the "Release User" function.)
If anyone has an answer to that (or a way to fix this bug regarding permission denied when a client is left open) then please let me know.
Lonnie