Try to quick edit your message once and hit alt+s, it will work as expected. Now try to do it again - "body message was left empty", just as SMF does.
Tracked: https://github.com/elkarte/Elkarte/issues/2238
https://github.com/emanuele45/Dialogo/commit/e10d02274ee7252e0efe4b4bc3f3f8baf5f9f99c