I have this exact same problem, unfortunately. It's incredibly odd, and I have absolutely no idea what's going on.

For the last few days (can't remember exactly when this started, maybe 3 or 4 days ago) I've been unable to join the office and de_dust deathmatch servers - even through the console -, aside from one time when I somehow got in during the afternoon two(? - RIP memory) days ago. That day I was unable to join in the evening. From restarting steam to restarting my computer to reinstalling CSS, just about nothing I do has worked. Really odd.