I have had this issue for a few years now, since Exchange 2000. I have since upgraded clean on to a new server. Did not have this issue with Exchange 5.5
When the client attempts to log on, the screen very often hangs with a little progress bar at the bottom. Sometimes you have to click "log on" about 20 times for OWA to load. I notice besides the browser progress bar, a file referenced is owaauth.dll, which OWA is attempting to 'open'
I have followed Microsoft's firewall configuration, opening all the appropriate ports for the front end OWA box (in DMZ with public IP), to reach the backend mail server and DNS server.
Any help would be greatly appreciated.