Port 1600/TCP to DC after applying KB893066 (and other Updates from May)

Hi!

On May 14 we have installed the new Windows-Updates 890923, 893066,
893086, 890859 and 842773 on our Windows 2003 Servers.

Since the restart of the Domain Controllers, all Servers and Clients
startet to use the port 1600/tcp to communicate with the DC´s (along
with 389, 135, etc..). for our DMZ-Server this is currently blocked by
our firewalls.

Since this day we experience an issue on the DMZ-Servers:

The logon process is much slower (at the "Applying you personal
settings" part). About 30 seconds instead of 3 seconds.

My questions are:

What is the Port 1600 for? Should we enable it on the firewall?
Why is the logon process so slow?

Thanks in advance.

Regards

Jürgen Winter