How to reset an iPhone photo library with all data and meta data
To answer my own question:

I felt brave and fired up a 3rd party tool that could access the iPhone filesystem. In the directory Media I renamed the directories PhotoData to PhotoData_old, DCIM to DCIM_old and Photos to Photos_old.

Then I restarted the iPhone. After reboot completed I checked the Media directory again. PhotoData, DCIM and Photos directory had been recreated. Photos was telling me: No photos found.

I deleted the _old directories and synced photos from a good and stable library to the phone.

The iPhone photos library is now usable.