Dear @neox and everyone else with a similar problem: Did you maybe change your desktop environment in the past?
I switched from Gnome to KDE some weeks ago (installed the packages indicated for KDE and removing all those mentioned for Gnome here: Install Desktop Environments - Manjaro)
As I was experiencing some minor issues though, e.g. system settings crashing when trying to select “application style”, I tested whether there were still orphan packages from Gnome around, so I ran
pacman -Qtdq | pacman -Rns -
This removed 124 packages, some by name obvious leftovers from Gnome, and freed almost 1 GB of disk space. And last but not least, now the open dialogue of keepassxc works (again).