Aaaaaaand... deleted the rant.
Because after trying again under a Wayland session I couldn't trigger the error again. It all worked as it should.
And as X11 is planed to be removed from KDE/Plasma in January, I won't complain now.
(ok, THEN I shall rant, because right now Gamepad support simply isn't there in at least FS-UAE and, if there isn't another driver selectable, in Retroarch as well under Wayland)

