Zentyal automatically sets user password expiry using Samba, to 365 day ( 1 year) on installation
There is no way of setting this age limit or removing expiry from the main Zentyal portal, in either commercial or community editions.
Run the commands above using sudo e.g.
sudo samba-tool domain passwordsettings show
sudo samba-tool domain passwordsettings set –max-pwd-age=0
Source : http://www.orion-online.com.au/zentyal-password-expiry/