Same thing here. I've done EVERYTHING that's been recommended in this and Steam forums: port forward, the masterservers file, deleting the blob and such... etc, etc etc... There's not a single fix I have not tried, honestly.
The game ran PERFECTLY until a couple of days ago, I don't know if there was a Steam update or what, it suddenly became screwed...
If this was actually caused by a Steam Update, it would be great to hear something from the guys at Tripwire, so I least I can stop pulling my brains out and wait for it to get fixed...