[itsme]

It's something I'm going to have to look into.

MobileSheets always tries to move files when the storage location is changed. Unfortunately, it's problematic to reset the settings with a default storage location and then pick the new storage location. You can manually edit the default.xml file to specify the storage location and then pick the folder when prompted at startup to avoid this scenario.

Mike

Why MobileSheets tries to move files that are not there is imho not worth investing effort, I assume it's a rare scenario and except taking time it doesn't do any harm.

More important:
Deleting default.xml did not change the behaviour. So I uninstalled MobileSheets and reinstalled it from the MicrosoftStore. After uninstalling LocalState was completely empty, I checked that. After reinstalling a new, empty mobilesheets.db had been created, so there was no issue with copying anything. I created a new msptest.db and imported EineReiseInsGlueck_F.pro as in my first post.
VERY STRANGE: transposing lowercase still doesn't work!
I have two computers pretending to run the same OS and the same MobileSheets version and they behave differently!

Solved. 
Text File Settings - Allow Lowercase Chords = ON was missing.
This is (quite logically) required for transposing and transcoding lowercase 'chords'.