When I upgraded my TFS 2015 Update 3 instance to TFS "15" RC2 (yes, still on a codename), a few of my users were unable to authenticate using Visual Studio. Authenticating through the web application worked fine, but Visual Studio would continually prompt for credentials.
TF30063: You are not authorized to access https://xxxx.com/tfs
The solution is to remove cached credentials from Credential Manager:
- Load Credential Manager from the start menu
- Click Windows Credentials
- Remove all saved credentials for your TFS URL.
- Reload Visual Studio and voila!