Following comes to us from Deepak Mittal.

Users may see this error in VS:

TF31003: Either you have not entered the necessary credentials or your user account does not have permission to connect to the Team Foundation Server at https://servername/tfs Click to Use different credentials link below, or ask your server administrator to add the appropriate permissions to your account.

In case this KB is installed, there are few possible solutions in case you face TFS connectivity issue:
o Disable kernel mode authentication
o Disable compression on IIS
o Force client to disable compression
1. Disable compression at IIS level as mentioned in below msdn article:
2. Disable kernel mode authentication at IIS as mentioned in below msdn article:
3. Disable the compression on TMG for TFS:
ForeFront TMG -> Web access policy --> HTTP compression --> Set it to disabled
4. Upgrade your TMG environment to 2010.
5. There’s a workaround from the client side for this issue, which is to set the registry key to not enable compression for this client (so client will not send Accept-Encoding header). So compression is part of the cause here, and it involves the HTTP API. Set the following registry key:
Key = HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\12.0\TeamFoundation\RequestSettings
Value = (EnableCompression, REG_SZ, False)