To have it immediately closed, because that is useless. That message means the server sends a 500 page or similar, i.e. TRASH.It means rtorrent can't parse the data the tracker sends back to it. There's nothing you can do, except file a rtorrent bug report.
I agree that it's the most likely cause, but unless you've proven rtorrent to be 100% free of bencoding bugs, then it could be a bug in rtorrent.To have it immediately closed, because that is useless. That message means the server sends a 500 page or similar, i.e. TRASH.