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.

Resolution:
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:
http://technet.microsoft.com/en-us/library/cc730629(v=ws.10).aspx
 
2. Disable kernel mode authentication at IIS as mentioned in below msdn article:
http://technet.microsoft.com/en-us/library/cc754628(v=ws.10).aspx
 
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)