This is because the binding is incorrect for the certificate. Make sure that this is the ONLY website in IIS before making any changes. 
(IIS 7.0)
1. Right click default website
2. Click "Edit Bindings"
3. Make sure there is a binding for  http (port 80) IP address should be "All Unassigned"
4. Make sure there is a binding for https (port 443) IP address should be "All Unassigned" and then make sure the proper certificate is selected under SSL certificate
5. Save- and error should go away and SSL certificate will Authenticate.
6. Do an "iisreset" in the run command to reset IIS

Note: IIS is not a Blackbaud product, and is not supported. This information is provided to help assist you in resolving an environmental issue.