This problem is caused for using a CDKey that is not supported for the Game/Server in FEAR Combat and FEAR (Included the Steam Version).
When does this happen?
Steam:
The Steam Version of the game doesn’t officially support Multiplayer but It works!
1) Get a FEAR Combat Community CDKey, registering at this site.
2) Install the FEAR Combat Patch, It will create the file once you type your CDKey ingame. You can Optionally Create the key.ini file manually and add your CDKey, but that’s not really needed, the patch takes care of this.
3) Play.
FEAR Combat:
1) Get a FEAR Combat Community CDKey, registering at this site.
2) Change your FEAR COmbat CDKey to the one you got.
3) Play.
If you still get this error, remember you can use your CDKey in SEC2 Servers, that have [SEC2] in the map name, but all the Servers registering at the new FEAR Combat Master Server are SEC2 :
FEAR Combat: Identify SEC2 Servers