Co-op in Gmod 9.0.4

[grimm2005]

hey,

trying to get a co-op server running for me and a mate to play the hl2 campaign/gmod in general but am getting an error when trying to create a server.

when i go to set up a server and select chapter 1 (or any) from the list, it starts to open and sticks on the first bar (establishing connection to server) and eventually crashes to desktop with an "Unable to load athentication library, Exiting..." error.

Can anyone help me or tell me what i'm doing wrong?

Thanks

 

[Operational]

http://support.steampowered.com/kb_article.php?ref=6579-QRTX-2246

 

[grimm2005]

Thanks for that link but after trying those solutions it still doesn't work