Whow - it is working.
Maybe one cannot use cloud drives - probably they try to be too private - even if I disabled all privacy.
That's a pity because cloud drives offer a lot of space.
Thanks Frank for your time - at least I have now an idea what was wrong and that it might work...

Probably eats up too much of the clouds bandwidth

I think you are right - Meanwhile I tried another Cloud service - the same problem

I think one has to forget to use cloud drives so far for html5 Videos.
I didn't get a link I could use in SD from

Google drive
pcloud.com
Degoo.com
Mediafire.com

If anyone is lucky please inform me directly at ergo@snafu.de

Make sure the link you're using is direct to the .mp4 file, sometimes this means you might need to tweak the URL a bit or adjust settings in the cloud storage to allow for direct media links.

I had a similar issue where videos wouldn't play in the HTML5 video element. Turns out the problem was with the way the cloud service provided the link—it wasn't a direct link to the MP4 file. I had to dig a little deeper and find a link that ended in ".mp4" to get it working properly.

Also, double-check the MIME type for your video. Your server might need to be configured to serve MP4 files with the correct video/mp4 MIME type. Otherwise, browsers might not recognize it as a video file.

If you're looking to edit your video or need to convert it, https://www.movavi.com/imovie-for-windows/ has some easy tools that might help. They're pretty straightforward for anyone not too tech-savvy.