So I was bitten by my very own mistake heavily.

Don't forget to correctly set the environment variables each time you fire up your self-compiled Kdenlive installed in a non-standard location ... such as inside your home directory. Otherwise, Kdenlive will come up but later crash in totally unexpected situations without any clear indication that it could not get at its qml files or other files it needs at runtime.

I've added a discussion topic to the KF5 wiki article: https://community.kde.org/Talk:Kdenlive/Development/KF5