I edited my app.config file manually, which did not seem to agree with VS. Even though the XML was valid, I received the message: An error occurred while reading the app.config file. The file might be corrupted or contain invalid XML. Despite this message the code will still run, but its just annoying.
This is a known bug, which I managed to get rid off by removing the project from the solution, restarting Visual Studio and adding the project again.