Thought I have to share this piece of knowledge. Although not in a commonsetup,
someone else may hit this trouble. I had a look through HMC support at IBM but
did not find anything similar.

I was upgrading HMC V6R1.3 to V7R3.1, all according to IBM manual. Saved upgrade
data to HD, saved critical console data to network, burned 2 DVDs with upgrade
and rebooted. After successful upgrade HMC started and presented me with a
screen 'The requested URL could not be retrieved - /prelogon.html'. I have spent
quite a while trying to find out what went wrong (even had to get` unauthorised'
root access to HMC by using Fedora rescue disc). What I found out at the end is
automatic proxy configuration script (taken from company proxy server, running
Fedora linux, squid and apache) by searching Mozilla Firefox cache file
(/home/browser/.mozilla/firefox/ki0egrt3.default/Cache/ directory). This proxy
configuration script had a mistake, assuming that URL pointing to 127.0.0.1 has
to go through proxy server. Thus Firefox was unable to get
http://127.0.0.1:8080/prelogon.html script required for initial HMC startafter
upgrade. I am not sure how Firefox found out about the proxy and how it managed
to get proxy configuration file (we got static IP for HMC and never configured
any proxy settings for it), but this seems to be something new introducedin HMC
V7. We always had same proxy configuration scheme on our network, and other
versions of HMC (starting from V4) never did that.

Alex