Opening a message or changing views in OWA 2003 logs user out

Most users in my organization have email access via Outlook Web Access 2003 (Exchange Server 2003 in a Windows XP Professional environment). I have a user who gets kicked out back to the login screen of OWA every time he tries to open a message or changes views (from Inbox to Calendar, etc.). This just started yesterday and he has never had this problem before.

I have googled this but have not found any answers so far.

Thanks.