I played through the single player offline campaign with no problems so I decided to start playing the add on campaigns. I played Warcelona with no problems but then I started playing Two Evil Eyes and got the message:
Disconnected from server: Server is enforcing consistency for the file: models/infected/boomette.mdl".
I also get the same message while playing Blood Tracks. Basically, I can't play these two campaigns which sucks. I haven't played any of the remaining add ons, Deathcraft, Haunted Forest, City 17 etc., but I suspect the same thing may happen with those too.