I tried installing qbittorrent from terminal as you suggested but it says it is already the latest version:
avin@Monty:~/Documents/gwimages$ sudo add-apt-repository ppa:qbittorrent-team/qbittorrent-stable
[sudo] password for gavin:
You are about to add the following PPA:
Packages for the stable ...

Thanks for explaining things, it is really helpful.

I was running version 4-something of Qbittorrent, but replaced it yesterday with the current Flatpak version (no sandbox or anything unusual AFAIK). Had no problem getting it set up otherwise, but this running an external program has never worked ...

Note that all the pastes from the post above are private, so I couldn't look at them.
:oops: I have checked and they are all public but they seem to be 'pending moderation' which I guess is the problem. The ones from my first post should be fine but the second ones maybe aren't available yet ...

Right, okay that makes sense - thanks.
Does /home/gavin/debug.sh work at all? How did you confirm this?
I believe it is working because when I ran it in terminal it generated six text files in the /tmp folder:
filebot.sysenv.txt https://pastebin.com/NzaS6FKH
filebot-sysinfo.txt https://pastebin ...

Apologies for having to ask this but I am at my wits' end. I have searched and read and learned a lot but I cannot find a solution that works for my system. I am new to Linux (sorry again).

Issue
Calling the Filebot AMC Script from Qbittorrent's 'Run external program-->Run on torrent finished ...