I could solve following problem:
On Windows 10 (20H1) after one of latest Windows Updates: Neither Cubase 11.0.30 nor Waves Plugins installed from current Waves Central 12.0.20 did anyhow complain. They appeared as if installed properly but when I selected a Waves plugin the slot was cleared after selection.
I found a hint here in the forum to reinstall Microsoft C++ Redistributables 2015-2019. This did not solve my problem.
I tried to install Microsoft C++ Redistributable 2012 (vcredist_x64_2012.exe) from the Waves programm folder c:\Program Files\Waves Central\resources\res\external\Redistributables\2012.
The installer offered me to fix the installation: Bingo - solved my problem after countless hours of investigation. Plugins now load properly as expected.
Perhaps this may help other users.