The ‘TestWindowPackage’ package did not load correctly

This issue is experienced while opening the test explorer or running all tests in VS. To resolve the issue try the following steps:

1. Close all instances of Visual Studio.
2. Delete all files in AppData\Local\Microsoft\VisualStudio\12.0\ComponentModelCache (Note that directory is located in Local.)
3. Restart, if that doesn’t work then start over with step 4.
4. From a Visual Studio Command Line do this operation: devenv /setup

The points 1 and 2 should fix the issue. But I found that sometimes more steps are required.

See also

• some more details about the issue on connect.microsoft.com