When video selected as background on one of the not primary monitors, I got only the first frames of every video before the next video showed its first frames.
I found that the cause of this behaviour lies in the audio source.
I had in windows 10 the output of dsf audio rerouted to another source (this is possible with just windows 10 education). After switching the audio back to the default everything worked as normal.
At the time the rerouting of the audio source is done, everything seems good, but the problem arises after a reboot.
If, at boot time, the default audio source is the same as the one set for dsf everything works. After boot you can alter the default audio source while dsf keeps functioning with its own audio source correctly.
I hope that I can help others with this problem.

Do I have to post this elseware, because it is a sort of bug?

Ok: steps to reproduce,
-set dsf to use video as background.
-use a map full of video's to make dsf use it as a playlist. I assume that all the video's are playable by dsf, if not, think of codecs.
-a Howto to change audio settings is here: https://www.howtogeek.com/352787/how-to-set-per-app-sound-outputs-in-windows-10/
-set the default audio to something other than you would like to use with dsf. So normal speakers would be fine.
-set the audio for dsf to something other than the default
-reboot

-problem: the video's in the playlist change to quickly, only the first few frames of each video is visible.

-solution/workaround
-1: change, when the problem occurs, the dsf audio to the default. After that you could change it back to the desired audio for dsf.
-2: change the default audio source to the same as what dsf uses. Both audio sources must be defined in settings, so dsf can't be set as default, but must be defined as somthing specific. Make the dsf audio device as the startup device with Audio Switcher. After boot I manually change the default from the dsf device to the normal audio device also with Audio Switcher (freeware). I'm able to do that with just one click.