Hello Frank Bauwens,
Let's use the steps below to boot your iMac into safe mode to see if that behavior persists from there:
1. Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.
2. Release the key when you see the login window, then log in to your Mac.
3. You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window.
You can also find those steps in this resource: How to use safe mode on your Mac
It can take a few extra minutes to boot into safe mode. Your screen may flash or flicker while in safe mode. Those behaviors are expected. Testing in safe mode runs a check on your disk, removes some cached files, and can help isolate conflicts with startup items. Once you've tested that behavior in safe mode, you can restart to leave safe mode.
Should that still persist, do you experience the same behavior with other video content, such as online videos?
Are you able to play those particular items using the TV app on another Apple device?
Let us know, and we'll keep moving forward based on those details.