Lowercase 'm' is still working here in TB52.3.0 on Windows 10. So it doesn't look like the shortcut has been withdrawn.

What's changed recently? What else might be consuming your 'm' keystroke?

I note that the keyboard shortcut article incorrectly suggests the uppercase 'M' (implying shift+m) is the shortcut; here on this installation that is mapped to an add-on's folder search.

Thanks Zenos, for your quick response. It doesn't work for some days before and after the OS update from today it works again. Surprise! By the way, I use the same setup. THX, I'm lucky