Quicktime Problem?

[moribundman]

I often download videos from youtube and use iSquint to convert those files into mp4 files, which, until recently, would play just fine in Quicktime. While I can still open the files, I suddenly don't get the video, but only the sound. That's also the case when I try to play mp4 files that previously played fine in Quicktime. All files play fine in VLC.

Any idea if this is a Quicktime problem?

OS 10.4.11
Quicktime 7.4.1