Mp4 video will not be loaded

Hi
I will move from my own Nextcloud Hub 6 (27.1.5) to an external one, like murenacloud.
Now I’m testing this and found a problem with mp4 files. I got:
“The media could not be loaded, either because the server or network failed or because the format is not supported.”
The same mp4 file is working on my own Nextcloud. Both Nextcloud instances are connected on the same PC.
The video I made with a Galaxy S20.
Does anyone have any idea what this could be?
Thanks a lot