After some debugging and finding the File Browser feedback thread here (File Browser support and feedback thread - #12 by JoOppen) I come to the conclusion that the issue comes from an incorrect implementation of xdg-open with jolla-gallery. I can reproduce the issue by opening a video file from the terminal via
xdg-open Videos/testvideo.mp4
Since i suspect the issue started with the implementation of firejail for jolla-gallery, I’d presume the issue lies somewhere here.
The only workaround I’m aware right now is to open videos from within the gallery app.
I have contacted jolla support to create a ticket in their internal bug tracker.