Cannot import mp4 files to iMovie anymore

A few weeks ago I was able to import mp4 files to iMovie. I've encoded the files with Handbrake first.

But now, I've downloaded a mp4 file (and then to be sure encoded this one too) but can't import it in iMovie.

How is this possible? They are all in the same mp4 format, but my last one doesn't work. It won't even play in Quicktime.. 😟

In the image below, you can see that the upper file can't be selected while trying to import it into iMovie.