Happened to me once too, i just closed grimoire and reopened it. i'm not sure how to reproduce it though and without us being able to reproduce it, not sure how we can fix or address it
Found a solution:
it can sometimes happen when the server restarts. i know this because i was watching my bot when i saw the server restart notification and i just let the bot keep running. when i came back a few minutes later to see whether the autorelogin on grimoire worked after the restart, it was exactly as the user described it: full black, nothing at all visible. i'm not sure whether this always happens after a restart but the timing was too close together to be a coincidence.