Now it works. Thank you.

Check what is set as default for MOV itself. Windows has both a global video player setting and a per file type setting.

The solution was to change the default video player in Windows settings and then change it back. Now it works.
However, it works only with mp4 files, not with mov. Is this normal?

I recommend either updating MediaMonkey to see if this is already fixed or try setting a different video player as default app, to see if that works for you.

Yes. They are .mp4 and the app is the default for .mp4

Is it set as the default App for the specific video file type(s) that fail to work?

I have 5.0.4.2693 (Portable mode)

I use MPC-HC as default player for both audio and video.

Which Build (Help > About) of MediaMonkey are you using and what is your default video player?

I tried on the [url=https://www.mediamonkey.com/forum/viewtopic.php?t=86639&sd=d]latest 2024 build[/url] and it opened VLC as expected for video files (and audio files are supposed to remain playing in MediaMonkey).

I have enabled Tools/Options/Player/Use default external video player, however it doesn't work.
The audio files continue to be played in MediaMonkey, while the videos simply don't start at all anywhere.
What can I do?
Thank you.