This shows that you are not using HTTP proxy or, as I wrote earlier, that another non-transparent proxy exists somewhere in
your network. In addition, you tell us that you added DNS but I don't understand where.
1 - Are you using DHCP? If yes, are you pushing Zentyal as DNS server for DHCP clients? If yes, is it effective (I mean configured as such at client level)?
2 - (again) what's about HTTP proxy settings?without visibility on this, any answer will be almost useless
What I suggest, now that you have explained faulty behaviour, is that you describe what is configured at Zentyal level.