I have done all of that before I even asked on this forum. Thank you for suggesting that anyways. I don't use extensions, it won't work in a private window, all caches cleared all cookies gone - same. It works on this macbook in Firefox and in Chrome perfectly. It works on the iMac in Safari (same OS version, same Safari Version).
So, once again: can someone who runs macOS 11.2.3 and Safari 14.0.3 on a macbook pro from after 2015 and who has subscribed to a udemy course confirm that they can watch that course with the video showing?
Because if they have the same issue I can stop digging through this box and wait for a fix. And if it works for them I am happy to hunt down what causes it on my machine/setup. (and please no more links - I have all the links I need, thank you)