Today I run into a strange error: In an .NET Windows Form I’m referencing a ConnectionString from Application Settings.
At 12:00 it worked.
Changed a line of code; recompiled; started over at 12:01.

The app came up with an error: “Unrecognized configuration settings UserSettings“, the debugger pointed me to the settings file.
Tried a lot. Searched the internet. Found the only working solution:

Navigate to your user’s application data folder, and delete all the config-files belonging to your app.