OWA "loading" after login - returns to auth prompt

I'll be as specific as possible on this as I've spent hours researching it and have come up dry with every MS Article and forum suggestion I've found:.

Clients running MS XP or 2k in IE (all versions are 6.0) are able to login to OWA but get nothing other than "Loading..." in all the reading panes.  After several minutes the login prompt reappears.  This *only* happens at *one* particular site (not a VPN site) and it happens to everyone at that site.   Other reports of this issue are random and inconsistent.  I cannot recreate this issue with any system I have touched personally.
--- Note:  There are absolutely no event logs on either the server or client.

Server:  MS Server 2003 (SE) (all updates applied) / Exchange 2003 / IIS 6.0 (single server exchange (no BE servers).
             No 'lockdown tool' or 'urlscan' is loaded (tho not relevant to 2k3, people keep asking me that)
Clients:  WinXp/2k - IE 5.0-6.0 (all updates applied) (mixed sp1 & 2)

Steps taken thus far:
1) http://support.microsoft.com/?kbid=280823 (no related subarticles are relavent)
2) http://support.microsoft.com/?kbid=831464 (no related subarticles are relavent)
3) Spoke with Network Admin for that site who swears up and down there is no proxy server anywhere on their network.
4) Added OWA address to 'trusted sites'
5) All iis directory perms are set properly.

Note:  I *can* login to OWA using the same credentials from another external (non-vpn) site.

This really seems like something on the property network backbone but I can't think of what would/could cause it.

Thanks in advance for all your suggestions,

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EE_AutoDeleterConnect With a Mentor Commented:
Because you have presented a solution to your own problem which may be helpful to future searches, this question is now PAQed and your points have been refunded.

accitAuthor Commented:

-- Issue resolved.

'client network' is running an 'application aware' firewall which was snubbing WebDAV.  Setup OWA for SSL and connected via https.  This successfully got past the firewalls WebDAV blocking.

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