CSS: Steam Validation Rejected

[NKTK9]

Hey, I'm currently hosting a CSS to play with my friends and what not; I've set up my server so its up and running. Everything is running ok (port forwarded, etc etc), but whenever I (or anyone else) tries to enter, "STEAM Validation Rejected" comes up. I've tried searching other places but got results so thats why I'm posting here.

Thanks for you help.

P.S I am not on a cracked version of steam/etc and have no bans on record.

[BehaartesEtwas]

make sure the firewall does not block a connection from srcds to the valve servers. (i.e. don't block any outgoing traffic from srcds processes)