I've deleted ClientRegistry.blob
I've deleted Config folder
I've deleted ALL of PVKII content and reinstalled.
I've rebooted the PC / disconnected from internet.
Most importantly, before I attempted any of these fixes, I ran Counter-Strike Source, and had absolutely no problem connecting to any server.
But when I try to connect to ANY server in PVKII, including creating my own, I get the error.
Account Used Elsewhere
This Steam account has been used to log in from another computer. To continue using Steam, you need to log in again.
EDIT: The steam beta broke support for mods. Do not participate in the latest beta, the one with the "big picture" feature. It was the cause of this error, and to undo the update if you already have it, just go into your "settings" and under the "account" tab, where it says beta participation, click "change" and select "NONE - opt out". Should restart and revert steam to stable build. Damn Valve with their shirleyty beta releases.