Apple acknowledges that this is (yet another) intel mac issue... (This migration has been a nightmare between this issue and SWsoft Plesk 8 not being intel Mac compatible either)
Oh, and disappointingly 10.4.8 server update did not fix this issue.
After rooting around the 'net I discovered a temporary fix...
Step 1. Flush the firewall by doing sudo ipfw -f flush
Step 2. Open the locked out Server Admin Console, click refresh and it'll unlock
Step 3. Quickly turn off the firewall and then deselect (uncheck):
a. UDP outbound and responses
b. UDP fragments
c. Serial number support (UDP 626) (a.k.a. serialnumberd)
Before re-firing up the firewall, double check the allowed ports so that you don't lock yourself out of remote desktop and/or your other services.
Step 4. Re-start the Firewall
If you watch the Active Rules, the Serial Number Support entry will "magically" replace itself as the top priority entry: 00001 0 0 allow udp from any 626 to any dst-port 626
Mac OS X v10.4.x does not "phone home" to Cupertino. The Serial number support (UDP 626) function searches for other rogue machines within the subnet using the same serial number. Apple's way of thwarting piracy -- It's sad that OEM's have to sabatoge their own software to ward off pirates.
Bottom line, if you're running UB Mac OS X Tiger Server on an Intel Mac you'll have to modify your firewall until Apple publishes a fix for the serialnumberd bug.