I currently have a TFS 2010 Server running on SERVER-1. On my client (MY-CLIENT) I have VS2010 running and have a workspace associating SERVER-1 with \\MY-CLIENT\\Development.
I had exactly the same issue: After moving TFS server to another machine, I couldn't map to a local folder in VS2012 on the old machine because it was still associated with an old Workspace that TFS denied all existence of. After many hours (and days) searching Google and trying different things, none of which worked (including all the "tf" commands, deleting the local cache etc), this is how I eventually solved it: