Is there a restriction for POST over HTTPS for IIS 7?

Question

I am creating a web service in PHP that processes data received from a Windows application through a SOAP request. A Windows application is a bit of a black box, but it generates a long stream of messages and then sends them as a single SOAP request.

Using HTTP, this works, and requests that are several MB in size work fine.

Using HTTPS, only small requests process large requests with an Internal Server Error 500 error and never go to the PHP web service.

The PHP application itself runs on the W2008 server under IIS7

Is there a parameter that affects this or is there a restriction on HTTPS requests?