> Thanks for your reply.
> I am afraid in maintainer provided /etc/fail2ban/jail.conf the backend
> setting is already set to auto by default.
>
> $ grep "backend =" /etc/fail2ban/jail.conf

Might be worth rerunning that command as
$ grep "backend =" /etc/fail2ban/*
to check one of the other config files hasn't overwritten that backend
setting.

However...

 > fail2ban                [31993]: ERROR   NOK: (38, 'Function not 

implemented')

Searching on the error message yields this:
https://dev1galaxy.org/viewtopic.php?pid=57444#p57444
which suggest a python library version / dependency issue.

$ apt install python3-pyinotify

seemed to fix it for that user. I have
    python3-pyinotify is already the newest version (0.9.6-5).
on mine alongside
    fail2ban is already the newest version (1.1.0-8).