We have an SBS2003 box running Exchange as normal, set up and working fine for Outlook over Internet clients and Direct Push mobile users. Outlook Web Access was fine until recently - now you log on as normal and see the mailbox displayed, but when you open a message or start a new message, you're prompted to login again (i.e. you see the OWA login page). Once you login again, you're fine until the next time you open a window, then you have to login again.
The really strange thing is, it only seems to happen every OTHER time the server is rebooted. I'm not 100% sure on this but am about to reboot to confirm.