Videos suddnely won't play anymore on iOS or Safari MacBook Air M1

It's not only me but also customers of mine report that they can't play videos embedded in Teachable (see https://courses.smartergerman.com/courses/a1-beginners-german/lectures/36194345 which is a public preview) on iOS devices anymore - no matter which browser and not on Safari on Mac Dektops/Laptops - but there Chrome works.

That wasn't the case 3 weeks ago but suddenly occurred. I assume it's an Apple or Teachable update but Teachable is not responding at the moment.

Does anyone have any idea here? I already deleted cookies and cache, deactivated experimental features, tried a private window and reset my DHCP settings all to no avail.

It shows either a 410 GONE error message on mobile or a 1-0004 (i.e. doesn't exist) on my Laptop.

Any help would be highly appreciated.

The intereseting part is that it plays fine on the backend in the same browser. See Loom here: https://www.loom.com/share/d0dc844a04074a4da2399b4e83e8288e

Curious to hear your ideas.

Thanks.