Invalid server

[Linchpin44]

I successfully installed the cod_co mod on my server, but later when I tried to log into the game as a player/client I got the error message Invalid Server.
The other mod I have, openwarfare, plays as expected when I log in
Also, I play regularly on another server, Raid Combined Operations, with no login problems.
My computer runs the COD4X version 21.1 of CoD 4.
What is the "Invalid Server" problem and how do I fix it.