I once noticed the same issue when connecting to the server with sq1 map. I didn't have the map, thus it downloaded the file, afterwards I got the error message too. But I can't remember if it kicked me out from classic-fortress or not. Anyway I just reconnected and I could play on the server.
Strangely it didn't happen when I voted another map that I didn't have. I had quitted the game earlier if that matters, so it wasn't in 1 session.
nt