Yes, Windows 10.
I don't think the version of the itch app (which, BTW, is still 23; don't know why it won't update) matters, because this happened when I launched the Nightly build from the Mannequin icon.
The bug should be fixed by now, thanks for your info! The latest version of itch app needs to be installed manually if I'm not mistaken. It has nothing to do with this bug, however it also has a quite nice new feature which lets you install both Mannequin's stable and nightly build on one machine (and auto-update them both).
Again, thank you very much!