Thread:Extension talk:WYSIWYG/WYSIWYG works by IP but not by URL

We have our wiki on a LAN, but for convenience to staff reaching it by VPN, maintain a special public domain which provides lookups to the LAN's private address space. So "wiki.specialdomain.com" resolves to, say 10.10.10.123 which is the wiki. If I reach the wiki at http://10.10.10.123 the WYSIWYG comes up fine. If I reach it at "http://wiki.specialdomain.com" it never comes up on the page after selecting "Edit". What could be causing this difference in behavior?