I have problems with rendering video files and most solutions I found related to deleting or editing certain files.
It's quite possible that there are many more that are also missing.
Now checking things, .kde/share/apps was restricted to access by root and I had no access with the user account I usually use.
I guess that means that the installation got really messed up, but removing Kdenlive and reinstalling it never got any change.
Can anyone help me to get a correct installation done?