" Unknown Steam error "

[Destrojah]

Always when I try to Play Solo on Killing Floor I get this " Unknown Steam Error prevent access to this " and I can't pick perks. But it works Normal in Multiplayer matches. I asked Steam Support and they told me that I should try it here.

Can someone help ?


Here is a screenshot:

http://i.cubeupload.com/uwpzFT.jpg