I just resolved an issue that started the other day where using OWA to access the public folders through a font-end server would result in a poorly-formatted toolbar and the word Loading… in the leaf pane. Access to mailboxes was just fine, and it would work when using the Basic client; only the Premium client would exhibit the behavior. Accessing the public folders directly via the back-end server worked just fine.
Because it wasn’t a content issue (mailboxes displayed fine) I could ignore permissions being the problem. Knowing that everything except the content comes from the front-end server (stylesheets, scripts, controls) I started focusing on file version mismatches between the front-end and back-end. Any patch that affects the rendering of OWA needs to be applied to the front-end first, and then to the back-ends.
I checked for hotfixes on both servers and they showed the same number of patches installed (based on KB article numbers). The cause started to show itself when looking at directory structures. The front-end server’s highest numeric directory in /exchweb was 6.5.7651.9, but the back-end server’s highest directory was 6.5.7651.25. Researching what hotfix has a build number of 7651 yielded KB 911829. This hotfix was released in April, but was rereleased in May as v2.
I patched the front-end server some time ago, but the back-end servers were only patched recently with this hotfix. I used Microsoft Update to install the patch on these systems. So what happened is that Microsoft rereleased the hotfix with newer files and a newer build number, but kept the article number under which it is published the same. Microsoft Update doesn’t detect that a newer hotfix is available when the article number remains the same. Downloading the v2 hotfix and running it on the front-end server created the 6.5.7651.25 directory and resolved the issue.
So this issue was caused by Microsoft not using good enough detection methods in Microsoft Update, and by me for not installing the hotfix on all the servers at the same time. The latter is easy enough to correct, but I doubt that the former will be fixed anytime soon.