I that server with no problems and so have several other people. For this problem to occur, it would have to fail to find the file in the manifest (which as far as I can tell never happens) and then the item_key.zip file would have to either be corrupt or have been modified since the game started. I don't see how this could happen without user tampering.
Generic solution: Remove all non official add-ons. Don't just un-check them, delete them.