If the option "wait for moderator" is selected for a BBB meeting in Moodle, then the meeting can't be joined from the Moodle android app even if a moderator is already present and the meeting can be joined from the browser. Reloading in the app doesn't change anything. The only thing that works is to choose "open in browser" from the app.

https://github.com/blindsidenetworks/moodle-mod_bigbluebuttonbn/issues/225