Domain Mapping Issue: Domain maps to main blog, not user's subdomain

I recently upgraded to the latest version of Supporter and Domain Mapping plugins. I received a new Support sign-up and the user attempted to map is domain to the server's IP address, which I specified on Super Admin -> Options. I have checked the user's A Record at his Domain Registrar and it is correct. He is attempting to map

As you can see, his mapped domain is being routed to the main blog site. What am I getting wrong here? What else can I check that might be causing this issue?