Server rejecting session login - Mountfilesystem error (444)

[sg_lostlogic]

When I go to start the TF2 Dedicated Server I get the following in the steam.log file:

CsComm Session Oct-09-2007 14:41:31.114 [5628] CreateSession(username_masked,444,0)=4 attempt 3 failed: Server rejected session login

I also get a popup on my screen about MountFileSystem failure (444) and no content servers available.

Any ideas?