Video function completely broken

Importing a video into Cubase 10.5 does not work. The thumbnail images do not show, and playback does not work (audio is fine). This is true for both Professional and Elements on PC and Mac. (VLC and Adobe Premier plays the videos with no problem).
I will now have to move from Cubase to Adobe Premier as my main workflow, which was not what I wanted to do. Can anyone help diagnose and fix the problem? I have already excluded the possibility of mismatched Sample Rate of the audio and project. This is not the cause.

Hi and welcome on the forum,

Please read through Video support in Nuendo, Cubase, WaveLab and Dorico article.

If you’re a pro user, maybe you’d be interested in my tools, it will make this process much smoother and easier.
There’s a 20 day free trial, so you can audition it to see if it fits your needs.
I recommend converting everything into ProRes 1080p.