INVALID DATA OR NO RESPONSE: THE RECEIVING PLUGIN MAY NOT BE ACTIVATED AT:

Hi, I'm trying to get WHMCS Provisioning set up and I think I'm close! But I've run into an error when testing. I'm using the following plugins:

WHMCS Provisioning
Domain Mapping
Pro Sites

http://billing.vaservics.co contains the WHMCS install.
http://vaservices.co contains the WP install.

When I try to create a new site via http://billing.vaservices.co I receive the following error in the WHMCS admin:

Module Create Failed - Service ID: 3 - Error: Invalid data or no response: The receiving plugin may not be activated at: http://vaservices.co

And the order status is Pending.

I have tried http://vaservices.co/?whmcs and received:
{"error":""}
so am happy the plugin is installed and activated.

With debugging on in WHMCS and attempting to manually Accept Order I can see the following in the Module Logs:

[1] => Array
        (
            [url] => http://vaservices.co/
            [content_type] => text/html; charset=UTF-8
            [http_code] => 400
            [header_size] => 547
            [request_size] => 975
            [filetime] => -1
            [ssl_verify_result] => 0
            [redirect_count] => 0
            [total_time] => 0.373499
            [namelookup_time] => 0.00136
            [connect_time] => 0.001609
            [pretransfer_time] => 0.001697
            [size_upload] => 726
            [size_download] => 9626
            [speed_download] => 25772
            [speed_upload] => 1943
            [download_content_length] => -1
            [upload_content_length] => 726
            [starttransfer_time] => 0.372982
            [redirect_time] => 0
            [certinfo] => Array
                (
                )

            [redirect_url] =>
        )

    [2] =>
)

I see a 400 Bad Request is being generated but I have no idea why. Any help solving this would be greatly appreciated.