VStudio MFC ISAPI dll , compatible with Windows Server 2012

I'm trying to port an old VStudio 2003 MFC ISAPI extension dll app to Windows Server 2012.

I've enabled the ISAPI extension on IIS manager, given permissions etc but get a 500.0 error when trying to run.

As the dll is quite complex, I built a new MFC  'hello world' dll in VStudio 2003 installed  on a Windows Server 2012 test server but still same result. I'be tried altering project configurations but no effect.

I'm wondering is there some compatibility issue with MFC ISAPI extensions and Windows Server 2012. 

Any thoughts would be welcome.

Dale