Ralf, I had the same issue but with an older version of the xdg-screensaver script. It's not the issue with the "missing" script but with the failure during running, which mpv doesn't distinguish. And actually the script seems even to succeed.
Try running xdg-screensaver reset echo $? in the console and make sure an echo outputs a 0. If not the script is supposed to fail. Regards, Andrey
