Left 4 Dead 2 - Engine Error ("transitioning player")

[714560]

I get this error/crash message at all times when loading a new chapter. Happens even on the maps where Coach isn't used.

"Engine Error
223/ - info_transitioning player: UTIL_SetModel: not precached: models/survivors/survivor_coach.mdl".

I've had also this error:
"Engine Error
223/ - info_transitioning player: UTIL_SetModel: not precached: models/survivors/survivor_gambler.mdl".

I don't use every mod when I play "seriously". I assure that I've no conflicting files enabled.
I'm not sure but the issue may have started after I installed some custom campaigns (Back to School, I Hate Mountains):

 

[Barbas]

Check out the download page for Back to School for help - it conflicts with at least one mod I use that other campaigns don't conflict with. Also, try verifying the integrity of your game cache and see if it works after that:

Spoiler

 

[Rayce Archer]

Note that generally a custom campaign won't give you problems unless you're playing the campaign. They're pretty self contained. Mod conflicts like above are possible but you should be able to run just about any campaign on vanilla without a hitch. Apart from the ones in the campaign, which are usually numerous