Hello, does s.one know what happened that suddenly I am unable to screen clips in the projectmonitor. The indicator in the timeline does not react. I tried to set back to original settings - nothing & to reinstall the program isn't possible as ubuntu-software does not delete the program?

It seems you have some “left over from a previous Kdenlive. Rename/delete kdenliverc see here: https://community.kde.org/Kdenlive/Configuration. Try with the actual AppImage.