Just for a little background info, I’m using Windows 2003 R2 and Exchange 2003 with all service packs and updates installed (last update today!) and I’ve been experiencing this “Loading” error for some time now.
First when I encountered this issue I tried delete and recreate all exchange’s virtual directories, change IUSR_SERVER account’s password, reset NTFS permission, reset virtual directories’ auth method and encrypt OWA using SSL, apply all hotfixes or in other words I’m doing everything I found on the web but no luck, I still stuck with “Loading” on my inbox screen.
So while I’m trying to find the solution, I force users to use the “reach” or “basic” client of OWA, util now…
Today I examined IIS log and found this entry
2009-06-28 13:25:26 W3SVC1 xxx.xxx.xxx.xxx GET /exchweb/6.5.7651.60/controls/util_view.js - 443 - xxx.xxx.xxx.xxx Mozilla/4.0+(compatible;+MSIE+8.0;+Windows+NT+5.1;+Trident/4.0;+.NET+CLR+1.1.4322;+.NET+CLR+2.0.50727;+.NET+CLR+3.0.4506.2152;+.NET+CLR+3.5.30729;+OfficeLiveConnector.1.3;+OfficeLivePatch.0.0) 404 3 50
notice there’s “404 3” near the end, that’s http status code and sub status code. I google it and found that “404.3” is “MIME map policy prevents this request.”
That sheds some light! After that I try to add wildcard mapping to the IIS MIME map by following this article and wholla!
OWA IS NOW WORKING AGAIN!
So that goes well, finally I change from wildcard mapping to JavaScript extension only (.js)

In short: add new MIME map for JavaScript (.js)
1. Open the IIS Microsoft Management Console (MMC), right-click the local computer name, and then click Properties.
2. Click MIME Types.
3. Click New.
4. In the Extension box, type .js
5. In the MIME Type box, type application/octet-stream.
6. Apply the new settings.
7. Open cmd then type “iisreset” then wait for iis to complete its restart sequence.

If you have any problem fixing “loading” problem, please leave me a comment.