Their are quiet a few Critical Errors. Not all of them have a definite fix but i do have the current one for the 1up servers. For some people the IJC mod will refuse to download and close your Killing floor completely leaving you with a critical error message. All you have to do is manual instal the IJC Weapons pack.
1. Download the weapons pack (
http://www.ijcdevelopment.com/projects/downloads/summary/4-white-versions/2-killingfloor-weapon-pack-white-v2-7.html)
2. Extract the Files
3. Go into your Killing Floor Steam folder (C:\Program Files\Steam\steamapps\common\killingfloor)
4. You have to go through and manually add the files to the right folder from the IJC Weapons pack.
Once you are done with that you should be able to play the game.