WPMU Permission Error
The other day I was at one of my sites where I run WordPress MU and encountered this error message:
You don’t have permission to view this site. Please contact the system administrator.
This appeared after the url after my login screen. The interesting thing was that the URL ending in ?c=1 counted up to ?c=5

Solution:
I quickly went to my MySQL GUI program (SQLYog) and went to the database for my WPMU install. The table labeled “wp_usermeta” seemed to be corrupted. After a quick repair of that table seemed to be the solution to the problem.
My guess is that since the “wp_usermeta” was corrupt WPMU was having a problem figuring out what rights I had in the installation.
Hope this helps!