FWIW We ran into the same error as Tobias today.
Same error, white screen on mobile, no top nav on desktop.
Happened on the home page (maybe on more pages, didn’t check).
Very long story 
short, the winning remark is here
Even though they link to the same repository, the plugin breaks when it is installed in a plugin directory that is not called discourse-user-notes.
So Tobias, I want to bet you’re old school and still called it discourse-staff-notes, right?