I've tried to connect to servers for games like Garry's Mod, Dota 2, CS:GO and none of them have worked I get a error message like this: 
I've tried port forwarding, virus scanning, disabling firewall, reinstalling ClientRegistry.blob and AppUpdateStats.blob, DMZ mode, allowing steam through firewall, verifying game cache and none of it has worked.
The weird thing about it is that only a handful of servers work but not all of them.
Best Answer
Try disabling/enabling Steam Beta participation and if that fails, try deleting appcache and even your Steam folder if you have to.