There is a note in the jail.conf file, where the backend is specified:

Note: if systemd backend is chosen as the default but you enable a jail for which logs are present only in its own log files, specify some other backend for that jail (e.g. polling) and provide empty value for journalmatch. See https://github.com/fail2ban/fail2ban/issues/959#issuecomment-74901200

There is an example for a jail only with a log file beside the journald:

[spam-filter]
enabled = true
port = http
logpath = /var/log/spam_log
backend = polling <--- backend set to 'polling'
journalmatch = <--- empty journalmatch filter
action = iptables-multiport