OWA access denied

We recently installed Exchange 2000 on a new server. We still have Exchange 5.5 running on the old email server and I am gradually moving mailboxes over to the new server. All mailboxes are functioning properly on the LAN and via POP3 but OWA is not working for everyone. Specifically, only Domain Admins can access their mailboxes via OWA so it must have to do with permissions on something but what I can’t figure out.