According to the documentation:

There are two approaches that clients can take to handle these signals. They can either connect a single handler to “action-invoked” or “query-variable” and examine the arguments to decide what action to take. Alternatively, handlers can be targetted at specific actions or variables by using the signal detail when connecting.

https://developer.gnome.org/gupnp/unstable/server-tutorial.html

This is incorrect. If you just connect with "action-invoked", it will never be called, due to this code:

                if (g_signal_has_handler_pending (service,
                                                  signals[ACTION_INVOKED],
                                                  action_name_quark,
                                                  TRUE)) {
                        /* Emit signal. Handler parses request and fills in
                         * response. */
                        g_signal_emit (service,
                                       signals[ACTION_INVOKED],
                                       action_name_quark,
                                       action);
                } else {
                        /* No handlers attached. */
                        gupnp_service_action_return_error (action,
                                                           401,
                                                           "Invalid Action");
                }

This requires that you listen for "action-invoked:YourActionName".

Either this check should be removed, or the documentation should be fixed.