New information (23.10.2018):
The problem could already be fixed.
Another update with release number 3.3.3 is already uploaded and should become available in AppStore within 1-2 days.
New information (24.10.2018):
The update 3.3.3 that resolves this issue is already online!
I'm sorry for the inconvenience.
Matthias Bauer, developer of SongBook+