I did the Outlook anywhere test on: www.testexchangeconnectivity.com
For my Exchange 2007 SP3 server with server 2008 r2.
Testing HTTP Authentication Methods for URL https://autodiscover.domain.com/rpc/rpcproxy.dll.
This part is the last thing and it fails with a 500 error.
I have outlook anywhere enabled with basic authentication and unchecked SSL offloading. In this dialogue box it also asked for the hostname and I entered autodiscover.domain.com (which points to the exchange server).
My OWA external hostname is https://owa.domain.com/owa