Upgrade Failure: "The installation and configuration of Team Foundation Server succeeded, however upgrading the data was unsuccessful"

If you received this message on the completion page of the TFS upgrade wizard, it means that the application tier of your TFS instance was successfully upgraded to the new version, but some or all of your team project collections were not.  In this case, you will be able to use TFS to create new projects and collections, but you will be unable to access your old content until you manage to successfully upgrade it as well.

The main reason for TFS being unable to upgrade team project collections is that their data was already either corrupted, incomplete, or stuck between schema versions.  You will need to contact customer support for assistance troubleshooting these data issues and getting your data back to a workable state.  However, there is also a chance that the upgrade process failed because TFS's network or SQL connection was lost partway through the upgrade.  Before calling support, you may want to double-check, and if necessary repair, these connections and then try re-running the upgrade on your project collections.  To retry the upgrade, remove all the non-functional team project collections from your new instance of TFS and use the attach command to attach the backups of these collections to your server.  Please remember to make a copy of your backups before attempting the attach so that you will still have a copy of your data in its original state in the event that the attach fails.