Another test - I have just now intentionally created some failed logins to the Roundcube webmail.
After a few of those, I could no longer log in at all to the webmail.
The fail2ban log file /var/log/fail2ban.log is still entirely empty, the service is enabled.
No entry in the log but I am being blocked!
The proof of this that when I say "service fail2ban stop", instantly I can log back into the webmail.
So it is fail2ban which blocks me.
Is there any other place where I should look for log files - I cannot find other logs but there should be, maybe this has been customized?