[DiapDealer]

I'm baffled.

Can you try changing your Sigil .desktop file (you keep saying "launcher", but I'm really hoping it's a file named Sigil.desktop) to explicitly use /usr/bin/sigil for its exec line?

Exec=env QT_QUICK_BACKEND=software /usr/bin/sigil %F

And see if it makes a difference?

Best practice would probably be to put your custom Sigil.desktop file in /home/ruben/.local/share/applications. You could create a desktop shortcut to it if you like, but all pre-existing menu shortcuts and desktop shortcuts used to launch Sigil would be overriden by the one in /home/ruben/.local/share/applications.

These .desktop files are designed to be in certain locations (/usr/share/applications, /usr/local/share/applications, and ~/.local/share/applications) and then have symbolic links created to point to them.