Video Block stops working – Library gives Media Error

When I first upload and post a video, everything works fine. I can play it in the Media Library, I can play it in the Editor, I can play it in a Preview and if I publish the post, I can play it on the website as well.

But after a variable time, all of that changes – on the website, in the Editor and in a preview, the block becomes unresponsive

; it shows the video controls, but it is not clickable.

In the Media Library, it reports this:
Media error: Format(s) not supported or source(s) not found

I am using the Twenty Twenty-One theme, but found the same problem with Twenty Nineteen.

I have no plugins activated.

Please help

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