After update to macOS 12.6.4 strange file permission change, plus QuickTime Player does not open downloaded mp4 files

FrankNStein wrote:

Hmmm, I don't quite understand, what this has to do with the problem. Some files had umlauts in their file name, but some did not. After 2 cold reboots QT Player finally opened those files and played them back.

There is a current known problem in that files with umlauts, accents, or other non-ASCII characters fail to open.

That is why I asked. Now it is clear this is a different problem.

FWIW, "Easy YouTube Video Downloader Express" may be the problem. I have no problems playing mp4 files in Monterey 12.6.5 with QuickTime Player.

Try booting into Safe Mode then restart normally. Sometimes that will fix anomalies.

How to use safe mode on your Mac - Apple Support

Since it does play, eventually, it is probably not a codec problem, but an mp4 does not uniquely identify a video file. Many codecs can be used to wrap it up into an mp4 wrapper. Apple has started dumping old codecs from the OS. Just because it is an mp4 doesn’t imply it should play in macOS.

VLC bundles almost every codec ever created, so almost everything will play in it.

I think, like dialabrain, that it may be caused by the codec used by the app you are using.

It may be something that is not optimized or fully compatible with the Apple Silicon processor.

Does the file name contain any non-ASCII characters, like Roman accented characters or some Asian language characters?