A clue. Try running
touch /var/log/asterisk/zulu_out.log
chown asterisk:asterisk /var/log/asterisk/zulu_out.log
And then restart fail2ban
A clue. Try running
touch /var/log/asterisk/zulu_out.log
chown asterisk:asterisk /var/log/asterisk/zulu_out.log
And then restart fail2ban