Strange, once I had that too. However it could be due to several reasons:
1. You have no "Guid" ID in the server, ask an admin how to fix this. (Update Punkbuster mostly is the case! Do /cl_guid in the server, to be sure it reads it right) Otherwise check if you have PBEnabled on your Profile.
2. When you have Windows Vista you need to "run as administrator" (ET.exe or shortcut) to load the game for the main user of the PC.
3. Invalid .pk3 files OR older .pk3 files, rarely happens and probably isn't the case. The server could get temporary data only for the current connection coming from you...
Try reason 1.