ASP.NET Site failed starting up...

This is a new problem. I've been using WebMatrix 3 for quite a while. It wasn't until recently that I've been experiencing "ASP.NET Site failed starting up, ..." error. This does not directly affect my website or its functions. However, I do think it affect WebMatrix built-in features such as auto-fill/auto-guess as I type.
I noticed that it seems off -- plus the error keeps popping up and in the way of buttons that I used at the bottom, especially when switching between files and database.

Any assistance would be greatly appreciated!