Today I had the problem that my Nokia N900 Mail for Exchange could not sync with my Zentyal server via Active Sync. After some research I found a solution in the Zarafa forum.
It consists of activating LOOSE_PROVISIONING in your z-push config.php.
If you are using Zentyal 2.0 this is located at /usr/share/z-push/config.php and you have to change the line:
define('LOOSE_PROVISIONING', false);
to
define('LOOSE_PROVISIONING', true);
now it should work.
Unfortunately I'm not sure if this gets overwritten during the next update!