Created attachment 2013 [details] gdb output Select message, hit Delete key -> message deleted. Select message, right-click + Delete -> crash Compiled per debug page. Running this way, right-click + Delete drops back to gdb; attaching "thread apply all bt" output.
It doesn't look like a Claws Mail crash, seems that the GTK+ library playing sounds¹ crashed, though the event to play sound is probably triggered by the delete alert panel. You may try to disable sounds on desktop and see if stops crashing, or, alternatively, disable confirmation of deletes to avoid dialog. The latter just to check it doesn't crash, not as a solution ;-) ¹ https://developer.gnome.org/libcanberra/unstable/libcanberra-canberra-gtk.html#ca-gtk-play-for-widget