I am not a native Linux user, although my Asustor NAS is Linux-based. I am currently using v2.3.9 (it's the version available in the Asustor app store).
I've tried the following things to fix this:
SOLUTION 1
I've tried removing the .pid file and restarting the app via SSH (but no luck).
SOLUTION 2
I've tried rolling back my version to v2.3.8 with these steps (but no luck):
1. Navigate to "[path]//Download//SABnzbd//log" and create the file called NO_UPGRADE:
2. Navigate to "[path]//.@plugins//AppCentral//sabnzbdplus//sabnzbdplus" and checkout a previous sabnzbd 2 version (I went from 2.3.9 to 2.3.

3. Stop and start sabnzbdplus using "AppCentral" (the Asustor app store)
Any further advice would be greatly appreciated! Thanks!
P.S. I wanted to post the specific commands and paths that I used above but the forum won't let new users post URLs and they recognise the paths as URLs.