Trying to get video player to work

Running NextCloud 16.0.0 and Video Player 1.5.0
I am unable to get any of the videos I’ve uploaded to play. It goes to a black screen and displays the spinning cursor. I have made small test videos via handbrake with the H.264 video and AAC audio presets, but no play and no information. Has anyone had any success with this player working? Any suggestions?

Also, I just noticed that in my test link,, it doesn’t seem to invoke the player at all and treats the file as merely a file to be handled by the browser. Is there a setting I am missing?

The mp4-files play all in the player on Firefox/Win 10. Just the *.mpg is downloaded and wants to be opened in an external program when I click on it.