I had this problem a while ago, and I recall seeing a couple of other posts
from people with the same issue. I never saw anyone with a fix.
Looking for something else, I've just come across this MS KB article:
Q246203 - XWEB: Configuring OWA Outside the Default Web Site
which may be of interest (it certainly was to me!).
Paul Taylor, Computer Systems Manager
WSP Group plc, London, UK