Sorry again and thanks for your patience @ThomDietrich.
I must have mixed some things from older threads. I followed your instructions at Z-Wave serial controller is offline
And now everything works fine.
Had to remove “console=tty0” from /boot/cmdline.txt and set “dt-overlay=pi3-miniuart-bt” in /boot/config.txt instead of “dtoverlay=pi3-disable-bt”.
I changed both at the same time before reboot, so I do not know if both or only one of the changes did the trick.
BTW: There is no /etc/inittab file in my system and therefore I skipped this step.