A clean install from scratch did the trick.
- Install 7.90.010
- start Kodi
- setup Kodi
- upload latest updates from milhouse
- append dtoverlay=hifiberry-amp to the lower end of config.txt
- reboot
- Setup Kodi audio options (44.1, 2 ch, hifiberry)
- enjoy
The "44.1" did it for me. So the default setting "optimized" isn't that optimal.