I was having the same problem a while back. Basically it's the Full 1.5 patch, especially when trying to join a server that has a Nothern Strike (NS) map running, even though I have NS expansion. The full 1.5 patch is corrupt as far as I can tell.
The fix:
-Install the game
-Install 1.2 full patch
-Install Northern Strike (if you have it/this has to be installed before 1.4 patch)
-Install 1.4 full patch
-Install 1.4 - 1.5 incremental patch
Hopefully this helps. The patches can found with a quick google.
Also, save the patches to a folder in case you need to reinstall at a later date.
