First, I would ensure nothing else is conflicting with that IP address, as that definitely could be a possibility. When you run arp -a on the firewall, what do you see? Also, IF you changed default web UI port, then it wont work if you have default filter or initial policy loaded, so please run fw stat and see whats there. If it says one of 2 I mentioned, run fw unloadlocal and it will most likely work. K, let me rephrase that...even with initial policy, web UI would work, but ONLY on port 443, nothing else. Also check below commands from clish.
Hope that helps.
[Expert@quantum-firewall:0]# clish
squantum-firewall> show web daem
quantum-firewall> show web daemon-enable
WebDaemonEnable on
quantum-firewall> show web ssl-
quantum-firewall> show web ssl-port
web-ssl-port 4434
quantum-firewall>
Andy