I recently got a mail about this problem. After a little research, I ran across the following article:
Problems Occur When You Open a Custom Form by Using the Favorites Folder
As a temporary workaround, if you access the public folder via the folder list rather than Favorites, you won’t hit this problem. Clearing your forms cache may also help (see the above KB for exact steps).
If you need a permanent workaround, you can call PSS for the hotfix. Brandon recently discussed how to quickly get a hotfix from PSS on the Exchange blog.
Lesson learned? Google is a troubleshooter’s best friend. On a related note, I may be giving a small talk at Tech-Ed this year about where to go for help with Exchange (I gave a similar talk at a previous MEC), and tips & tricks for how to maximize searches with google are part of the talk. Google rarely lets me down, what did I ever do without it?