The latest official AppImage is no longer working under ZorinOS 15.3 (in my case)
because of the fact that kdenlive suggest to use ubuntu 20.04 if file import of mov or mp4 files is no longer possible.
If you see black and green stripes in the video preview windows instead of a usual black preview screen than the only thing that helped was to downgrade to the AppImage 20.12.2
See also this post
https://forum.kde.org/viewtopic.php?f=265&t=170370&p=443655&hilit=20.12.3#p443655
So it is nescessary to upgrade ZorinOS to a base of ubuntu 20.04 otherwise next KDenlive Versions will no longer work under ZorinOS.