I had a user account who’s mailbox would not open from the Outlook Web Access.  After some poking around on the web and trying several things the following fixed it:

1) Open Active Directory Users and Computers
2) Enable Advance features from the View Menu
3) Find the user and click Properties, go to the Security Tab
4) Click the Advanced button
5) Check the box next to "Inherit from parent. . . "
6) Click Apply and OK 2x

That fixed it for this user.