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Here's a list of the most frequently encountered connection problems and what to do about them. Start at the top and follow it in the order indicated.
Verify that you have required permissions.
If the errors that you receive indicate read-only or blocked actions, you might not have permission to act on the data.
Verify that your computer is connected to the network and can access network resources.
Verify that TFS hasn't been taken offline. Talk with your TFS administrator.
Verify whether your team project has been moved to another team project collection in Team Foundation Server. If it has been moved, you must create a connection to the new server name.
For additional troubleshooting tips, see TF31002: Unable to connect to this Team Foundation Server.
Connect to a TFS with Secure Sockets Layer (SSL) configured
If you connect to a TFS instance that has SSL configured, then you'll need to install a certificate and clear the client cache. For details, see Set up HTTPS with Secure Sockets Layer (SSL) for TFS, Configuring Client Computers.
Clear the cache on client computers
When the on-premises TFS configuration changes, such as when moving or splitting a project collection, you might have to clear the cache.
Log on to your client computer for Team Foundation by using the credentials of the user whose cache you want to clear.
Close any open instances of Visual Studio.
Open a browser, and browse to one of the following folders, depending on the operating system that is running on the client computer:
Windows 10
Drive:\Users<i>UserName\AppData\Local\Microsoft\Team Foundation\6.0\CacheWindows 8
Drive:\Users<i>UserName\AppData\Local\Microsoft\Team Foundation\4.0\CacheWindows Vista or Windows 7
Drive:\Users<i>UserName\AppData\Local\Microsoft\Team Foundation\2.0\Cache
Delete the contents of the Cache directory, including all subfolders.