The problem has been solved, just follow these steps suggested by philipp273 on Apr 8, 2018:
philipp273Level 1 (9 points)
by philipp273 Apr 8, 2018 4:05 PM in response to VimalGanpat
So until Apple fixes this, here is a workaround: Downgrade to iTunes 12.7.3! It definitely is an iTunes 12.7.4-issue. After downgrading to 12.7.3, everything is working again.
So you have to manually uninstall 12.7.4 and download 12.7.3 from Apple (just google for the version number, you'll find it).
Problem is that you have to go back to an auto-saved iTunes Library file from when you had 12.7.3 installed. So after installing but before starting 12.7.3 you have to first rename the current "iTunes Library.itl" file to something like "iTunes Library 12-7.4.itl" and then go to the "Previous iTunes Libraries" folder and take the presumably latest autosaved itl-file and move it one folder up and rename it to "iTunes Library.itl".
(Of course this means that you'll lose all changes you made to your iTunes library since updating to 12.7.4)