Files not running after MacOS upgrade

[Ambitopia]

Have been using Avenue 7.13 on a MacBook Pro running MacOS 12.6 for a long time without problems. Upgraded Avenue to 7.14.2 couple of weeks ago, still no problems. Also upgraded to MacOS 13.3, no problems. But after upgrading to MacOS 13.3.1 some clips in my project will not play to the output. The clip seems to be running in the Clip panel (the transport control on the timeline is moving), but it doesn't show up in the Preview Monitor or the Composition Monitor. The clips are playable in Quick Time Player, and in Finder's quick preview, however. Clips that are rendered, or converted to DVX plays ok. Alley recognises the files and convert them ok, but the file sizes will be 12-15 times bigger, so I try to avoid converting unless it is necessary.
Any ideas? Can MacOS 13.3.1 be the culprit here?

[Zoltán]

what kind of files?

[Ambitopia]

.mov files.

[Zoltán]

Which codec is inside the movs you have?