Click image to enlarge
This problem ocures due to changes in Windows Vista and Internet Explorer 7 where dynamic HTML Editing ActiveX control was removed from Internet Explorer.
To solve this problem you need to update your Exchange servers with "Update for Exchange 2003 (KB 911829)". Note that you have to install Exchange 2003 SP2 before you can install this update.
Related KB article: You receive an error message when you try to perform any editing tasks, or you must click to enable the compose frame in Outlook Web Access
Important: Remember to always update your front-end servers first!
If you update your back-end server first and there is change in OWA functionality, clients will most likely not be able to use OWA. In the past these errors presented themselves as “Loading” text in OWA that never finished loading.
If you update your front-end server first, server will know about changes in functionality and will serve clients with working OWA. Now you can take your time and update all your back-end servers when you find time ;-).