If you try to use the Mastodon Android app, beware. If you already have an account, you cannot simply login. You must register for a new account, then pick the server you already use. THEN you login using those login credentials. Only then will the app work properly. You cannot login to the app as a current user, until you've done this. Don't ask me why, nor ask me why the initial login process is so weird. But that's the way it is.