Is there a limit to POST over HTTPS for IIS 7?

I am building a web service in PHP that processes data received from a Windows application through a SOAP request. The Wind开发者_StackOverflow社区ows application is a bit of a black box but it generates a long stream of messages and then POSTS them as a single SOAP request.

Using HTTP this works and requests sized in multiple Mb's work fine.

Using HTTPS only small requests work larger requests fail with a Internal Server Error 500 and never make it to the PHP web service.

The PHP application itself runs on a W2008 server, under IIS7

Is there a setting that influences this or is there a known limit to HTTPS requests?