Figured I should drop a note about this. I'm working with EWS 1.2.1 to communicate with Exchange 2010 and had the error "The response received from the service didn't contain valid xml.". Spent a couple of evenings trying to figure this one out. I recreated the EWS virtual directory, checked all rights, added SP2 and followed most available guides; nothing worked. Finally I figured what it was. There was a Host Name (a FQDN name) set to one of the Site Bindings. Removing that solved this issue.
This is how my finalized config should look