Hello, the problem is solved, thanks to debian-user@???

the problem was the programm call

mpv is not a gui-programm. In the mpv-manpage is mentioned the option
"pseudo-gui".

So I changed the programm call from this:
mpv "$VIDEOFILE"

to this:
mpv --player-operation-mode=pseudo-gui "$VIDEOFILE"


Wich works for me in Beowulf
Thanks Frank