samba - Jul 2016 - Samba PDC - NTP time sync don't work with Windows clients

After updating our Primary Domain Controller from Ubuntu 12.04 / Samba 4.1.17 to
Ubuntu 14.04 / Samba 4.3.4,
all Windows clients can't synchronize the time from our Domain Controller
anymore.
We are using the NTP configuration from the Samba Wiki.
(https://wiki.samba.org/index.php/Time_synchronisation)
When i run ntpdate -q pdc.mydomain.lan from a Linux box, time synchronization is
working fine, but Windows clients are getting a timeout.

I have used tcpdump -i eth0 port ntp on the server to see whats going on.
When using ntpdate -q pdc.mydomain.lan on the Linux box tcpdump shows that the
NTP server is answering, but
if i run w32tm /resync on the Windows client the server don't respond to the
NTP client request.

Can anybody help me to fix this problem?

Thx

Julian