OWA on Exchange 2003 gets "unknown error 400" when open up public folders

are you hosting a replica of the public folders on your OWA machine?

does it work in Firefox?

Are you seeing any other errors in the application logs?

Yes we have a replica hosted on the machine. The OWA works in Firefox because it connects in basic mode (without WebDav extension). And no errors in Application logs.

what version of IE?

and is this happening internally? externally? or both?  Running a proxy server? and, if so, are you by-passing it for internal addresses?

Check this out

http://groups.google.com/group/microsoft.public.exchange2000.setup.installation/browse_thread/thread/51c8951aca0cf48c/f7cdf1b1b103ae18?lnk=st&q=owa+public+folder+firewall+400&rnum=2&hl=en#f7cdf1b1b103ae18

You might be having issues with your firewall not passing the http-webdav-extension correctly

We are running IE7 and I've tried previous versions but no luck. The error happens internally and the webdav extension passed through for the initial OWA page (the first page when you logged on), all my mailboxes are available and accessible. However, we are using Trend Micro Officescan 8.0 and ScanMail on the exchange backend servers. I tried to do some research on the Trend Micro side but no luck. And Ive also tried to open OWA on the front end exchange and still got the same error. So& unless anybody got a better idea whats going on, Im not sure it is the firewall.

on your IE settings, have you defined the internal addresses that the proxy shouldn't be used to access?

Yes, "Bypass proxy server for local addresses" is configured through GPO.

try move a machine into a gpo that doesn't have anything defined for IE in the GPO

I've seen GPO's malfunction in the past