SO frustrating, when you download a YouTube 4k mp4 video and try to insert it in the timeline on Pro X it says cannot play Quick Time, I have Movavi Video Converter Premium and have tried various converting options but when I try to use it in Pro X it won`t accept and quotes QuickTime,.... Any ideas would be welcome.
Do you have QT installed on your PC? The last version for windows is QT 7xxxx. If you have, then it may be that the video uses a professional codec or a later codec that QT 7 can not play.
It would help us if told us which version of Video Pro X you are using and analyse the files with Media Info utility and copy and paste the text output to this post.