Steam unable to authenticate to server - L4D2

[Ph?bus]

So, I've done pretty much everything I can think of to try and fix this.
I've verified the cache of the game,
I've defragged the game,
I've updated Steam,
I've deleted ClientRegistry.blob,
I've updated video drivers,
And it still kicks me from servers because Steam is unable to authenticate the user to the server. This has been happening for the past couple of days and is infuriating. :facewall:

Any help?

 

[NixJedi]

I take it you've spoken to Steam support (since they produce the game and client that runs it)?

 

[Ph?bus]

Not yet. I just verified the game and defragged it again, so maybe that will help.

 

[Simon]

Blame it on the server? Sounds like it to me