Comment # 10 on bug 1218653 from Fabian Vogt

(In reply to Marcus Rückert from comment #9)
> confirmed. so maybe something in the KDE env that makes this work? some
> environment variable set? The errors above are from a gnome wayland session.
> 
> Fabian any idea about this?

Not really. Due to compositor bugs, Qt does not use wayland by default on
GNOME, but then also fails to connect using X11. With your
QT_QPA_PLATFORM=wayland override it gets further, but then:

(gmic_gimp_qt:216028): Gtk-WARNING **: 14:14:51.495: cannot open display:
wayland-0

implies that the process also uses gtk/gdk internally, but that one fails to
connect. So Qt is unable to connect using xcb and GDK is unable to connect
using wayland?

Using QT_LOGGING_RULES=*.debug and/or GDK_DEBUG=all might help figuring out why
either fails.

You could also try to set XDG_SESSION_TYPE=x11 or XDG_CURRENT_DESKTOP=KDE to
check if it makes any difference.