A user created a test (qti 2.1) with an essay element and tried to embed an externally hosted video through the question editor, but the video isn't playing at all.
I had a closer look at it and was able to reproduce it. Use the question editor and open the dialog to add a video. Paste the URL to the video into the address field and set the width and height. Although this would work inside of a single page for example it fails here. It looks as if the url to the mp4 (https://domain.tld/video.mp4) gets the url of OpenOLAT prepended (https://olat...) - see the attached console error.