Thanks for sharing your solutions.

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Btw, custom command blocks the WinSCP UI until completed in general. The Notepad does not block sometimes, as it is single instance applications (so if you already have an existing instance, the new instance just passes the file to the existing instance and quits). That's likely what happens with umpv.exe too.

I downloaded the version you're using, and it really can't play videos via a WebDAV URL.

This made me realize that different builds might behave differently.
I just downloaded mpv-x86_64-20260601-git-44a9b03.7z, and it can play videos from a WebDAV URL. However, it takes over the foreground of WinSCP, preventing me from continuing to use WinSCP; this doesn’t happen when WinSCP calls notepad.exe.

The version I initially used came from https://github.com/hooke007/mpv_PlayKit/releases which includes some useful tools.
When WinSCP calls the mpv.exe from mpv-lazy-20260510-noVS.7z, an error occurs. So I wrote a wrapper program in Go, and when WinSCP calls this mpv.exe through it, it works fine.

Finally, I downloaded mpv-lazy-20260510.exe and found that the extracted umpv.exe works very well (mpv.exe would take over the WinSCP foreground). (umpv.exe is a single-instance version of mpv.exe; this feature doesn’t matter to me.)

Conclusion: Although I don’t understand why some programs work fine on their own but can’t be properly invoked by WinSCP, the issue described in this post has been resolved for me. I hope I won’t encounter a similar problem again.

I have downloaded and extracted mpv-i686-20260601-git-44a9b03.7z to a random folder – and it seems that WinSCP can run it. I actually was not able to make MPV to play file from WebDAV (even when executed standalone from command line). But the player starts (both standalone and from WinSCP).

Can you share more details how your setup might differ?

a free, open source, and cross-platform media player
https://mpv.io/

What is "MPV"?

Windows 11 25H2
WinSCP 6.5.6
custom command:

Symptom: An error window pops up, displaying an error message roughly identical to the log shown below.
log: