Hello Paul, in that case I guess kicad is really trying to leave the process for some reason.
That reason is maybe written to stdout. Therefore maybe you can run kicad from a terminal and attach its output. Probably that could give any hints. Otherwise maybe it is related to the used desktop environment, if xorg or wayland is in use, or the used project - do you observe this behaviour if you just open one of the templates? Kind regards, Bernhard