Another post on fail2ban reminded me of a problem I had in Italy, when the ADSL connection kept dropping, and only came back on re-booting. (I solved the problem in the end by getting a Billion modem/router in place of the no-name one supplied by Telecom Italia.)
It seems that if there was no internet connection, fail2ban hung for an inordinate length of time, possibly for ever, when shutting down. I found I had to stop it separately, before shutting down or re-booting.