Hi,
I have the same problem, also with other Ports...
I had to deactivate the Firewall from within Zentyal to get things working.
Since my server runs behind a Firewall that only passes a few Ports to the server it is not a huge risk.
I deployed fail2ban with a modified jail-script to block intrusion-attemps and everything works fine.
By now, no intruder came through my firewall and manages to bypass fail2ban ;-)
Regards
Thomas