To solve this problem you have to clean the cache folder on your machine.
- Press Win + R together to open the Run window
- Type in %UserProfile%\AppData\Local and click on OK
- Navigate to the Microsoft\Team Foundation\6.0\Cache folder and delete all contents
- Restart Visual Studio and Re-Open your solution.
- The error message will disappear and all should be working again.
Image Credits: Photo by Isis França on Unsplash.