Client authentication error when starting Visual Studio 2015 SP3

Solution 1:

In the top right corner of Visual Studio, you will find a colored box with your name or your initials inside. Click the down arrow and then Account Settings.

In the opening window, please click "Sign out" in the upper left part of the window.

After a restart of Visual Studio and signing back in, the error should be gone.

Solution 2:

You can disable the auto connection on startup via the following steps:

1. Run "Developer Command Prompt for VS2015" as Administrator.
2. Run "tfpt connections" command from the developer command prompt.
3. Uncheck "Automatically reconnect to last server on startup" option and apply the settings.

Note that you will get 'tfpt' is not recognized as an internal or external command, operable program or batch file if you don't have TFS Power Tools installed. You can install them from Microsoft Visual Studio Team Foundation Server 2015 Power Tools.