Fixed. This was due to the backend in use (gamin) which doesn't work correctly under Debian. For an unknow reason the backend was set to auto (gamin so). Once I've set it to polling, fail2ban has started correctly.
Can you send me please a backup of your jail.conf file?
BTW: You should put your conf in the /etc/fail2ban/jail.local file instead of /etc/fail2ban/jail.conf. This allow to not lost your configuration when fail2ban is updated.
EDIT: jail.conf file restored (I've copied its content into the jail.local file, changed the backend to "polling", and then, restarted fail2ban). All is ok now.