Also having the same issue. My SAB version is: 3.0, running this via a DynDNS address, through NGinX to my local NASes (Two installs of SAB). Also have a custom base URL (via NGinX) for each instance, as they both run on the same ports, but on different devices, so can't pass both through the router otherwise. Used to work until about 3 updates ago. Was running the Python 2.x version(s) of SABnzbd at that time. Then all of a sudden, they both fail to update or display ANY queue items. The download speed portion of the nzb360 Queue UI and 'paused / downloading' status labels both work, but nothing in the queue is shown.
Sonarr, Radarr, Lidarr, and Deluge (2.0.x) are all set up the same manner, on the same devices, passed through NGinX the same way and nzb360 has no problem displaying the relevant content for those apps, just not SABnzbd's queue data.