When I run Cadabra_2.5.6_x86_64.AppImage from a terminal I get the following error messages:
Gtk-Message: 12:54:41.239: Failed to load module "canberra-gtk-module"
Gtk-Message: 12:54:41.264: Failed to load module "window-decorations-gtk-module"
Gtk-Message: 12:54:41.264: Failed to load module "colorreload-gtk-module"
but it seems to keep running, although with fonts that are not my usual choice.
Also, (not necessarily at startup) I get messages on the terminal saying
cadabra-client: connection to server on port 33555 failed