OWA multiple authentication prompts

Hi! I have an annoying problem. Out of the blue, (this is the first time in the 4 years it’s been running), my Exchange 2000 server is prompting 4 or 5 times for logon credentials when users are attempting to access their mailbox via Outlook Web Access. I don’t have ISA, and this is happening whether you are inside the firewall or outside. I’m having a tough time finding any information on this on the web, so any assistance would be greatly appreciated!