Resource Desynchronization Detected - Cant join any server

[Cappe!]

I get following message every time I try to join anyone's server, and my friend gets it when he tries to join my server.
It says following:

"Resource desynchronization detected Playing with desynchronized resources could lead to out-of-sync error"

I readed some topics and links from net, they said to do a clean reinstall, it didn't work at all: I and my friend have reinstalled game 2-times and we haven't been using any mods.

[Cappe!]

Fixed my problem, I registered the game on steam with the multiplayer cd-key and installed game via Steam. After that i could join multiplayer matches.