RPC over HTTP on SBS 2003 Premium

We opened a new office in another state and I am trying to set up Outlook RPC for it. The server created 2 certificates (sbscert for local and isacert for remote), so both, local and remote users, get valid certificate messages when they are connecting to OWA/companyweb.
The only problem that remains is that for some reason I cannot authenticate, Outlook (2007) just keeps asking me for my username and password (never brings an error message). I set up the client according to the instructions on the RWW. I use the wizard in ISA 2004 to publish the RPC.

What else can I try to get this working?

Thanks in advance.