I had a very similar problem and the instructions from a tech at Microsoft fixed it for me:
- Close all instances of Visual Studio.
- Open the Task Manager and check if any TFS Services are running. Select each of them and click on End Process Tree.
- Browse to the folder below and delete all the contents and folders in %LocalAppData%\Microsoft\Team Foundation{version}\Cache
- Go to Control Panel -> User Accounts -> Manage your Credential -> Windows Credential, select the VSTS URL to remove it
- Then go to "C:\Users\USER NAME\AppData\Local\GitCredentialManager\tenant.cache" and delete it
- Also go to "C:\Users\USER NAME\AppData\Local.IdentityService" and delete it