I am one of the administrators of the forum https://marktplatz.bewegung.jetzt
We noticed the following problem in release v1.9.0.beta10 +5 of which I am not sure if it did already exist in older releases.
At least we have not noticed it before.
In the preview when creating a new post a picture is shown
but when it is posted it does show the error, that no video with a supported format and MIME-type could be found.
here for your information the link posted: Tabu Tempolimit - 3sat.Mediathek
Is that perhaps related to this problem here?