Very often when trying to send a message with an attachment (experienced with compressed photo and with raw file), I often get "#400: Unknown error, see logs for details". Sending the file a second time always works afterwards.

I have no idea where I'm supposed to find this log. There's nothing apparent in the hamburger menu, help text, settings, FAQ, readme on GitHub, Play store description, … This really needs to be improved, it should be available in multiple of these places.