Banned from FEH

Get a new phone instead

Rest in pieces.

I got that exact same error message a couple days ago. It could just be a connection issue.

Had the same thing today, but it was my internet connection

Did you receive external confirmation that you're banned? I ran a Google search on the error code and I found some potential answers including https://www.perfectly-nintendo.com/fire-emblem-heroes-comprehensive-list-issues-possible-fixes/