ok Lightweight Directory Access Protocol error can have a slew of reasons. Eliminating the limitations of Small Business Server as the source of the problem, next would be communications between the SBS DC and the new member server. Joining a new member server to the domain, running the AD wizard and you get the LDAP error, I would suspect the problem is communications. SBS 2003 comes in a couple of flavors. Standard , Platinum, e.g. without ISA server and with ISA server. If you SBS2003 setup is with one nic AND you have ISA server, I would look to see if ISA server is interfering with communications. If you have ISA server, one nic and didn't create Packet filters allowing LDAP they will get blocked. IPspec authentication is another area I would look at.
I would suggest setting authentication on the LAN without IPsec, just use basic authentication and see if the problem clears up.