Ah, this is due to the auth being required for both 80 and 443 connections. The thing with it is this... when that happens you are technically authenticating rutorrent and the dashboard, then it forwards you to 443 connections wherein you are required to authenticate on a secure connection.
The dashboard is stored at
/srv/rutorrent/home which is due to how we hook in some essential rutorrent magic to make the dashboard so very amazing. Therefore, port 80 is requiring that authentication be submitted before forwarding to a secure port by way of rutorrent.
I have actually thought about turning off authentication on port 80 and just passing entry over to strictly https headers... but then people that demand using rutorrent without secure protocol get stiffed.
This can technically be removed by anyone that chooses and is using https (which is preferred) by editing the default-ssl.conf for the :80 virtualhost. Since you're using Lets Encrypt you can add a permanent redirect that will only require authentication once for both 80 and 443.. since it's imediately forwarding the headers to a secure port.
Add the line
Redirect permanent / https://DOMAIN add the end of the :80 vhost such as the example below.
Options Indexes FollowSymLinks
AllowOverride All AuthConfig
Allow from all
SCGIMount /quickbox 127.0.0.1:XXXXX
Redirect permanent / https://DOMAIN