Well... Hmm.. As I said, I am also still learning. But from what i've heard, if you mess with the standart LDAP port, your gonna have a bad time. Im afraid I cant help you on your specifik problem, but try to do som microsoft/google searches on errorcodes and/or parts of the problem description. I have in previus migrationtaskes, gotten alot of help, just from typing in part of the problem description in google and running the hits through.
My novice problem possibelities.
1. Ex2k3 can ONLY run on a 2000AD if SP3 or hight is installed (or of cause 2003AD).
2. Considering that the problem is the connection between the servers, my guess would be that it eighter LDAP, or you messed up some way along the path.
I sorry i cant me of more help, but try Google and Microsoft and see what it brings.
About IMs, i only use MSN mess. My addresse her is MJe(at)Doms.dk. But bear in mind, this is, also for me, a new book and i only read the introduction.