TF250061: The following site could not be attached to Team Foundation Server:
The site is already associated with a team project on an installation of Team Foundation Server with a different ID.
When a TFS site collection is associated with both TFS 2010 and TFS 2012/2013
This problem occurs only when you have migrated to a newer version of TFS while your collection is still attached to older version of TFS. You can fix it by going into the old TFS installation via Team Explorer in Visual Studio, and re-associating the project with it's project portal.
Select Portal Settings in Team Explorer and disassociate from older version, and then attach to the newer TFS version.