ASP.NET: WebResource.axd call 404 error: how to know which assembly/resource is missing or responsible?

I get a 404 HTTP status error (not found) on a specific WebResource.axd call inside an ASP.NET 3.5 (AJAX) web application. I guess the error is thrown because a specific referenced assembly is missing in the bin folder/GAC. But I don’t know which, since the page which requests the resource is very complex (I’m using third-party controls and ASP.NET Ajax.)

Compress and Minify WebResource.axd and ScriptResource.axd in ASP.NET 4 WebForms

I have been trying for a week now to reduce the number of requests on our web application but I can’t seem to combine the .axd files. I got a suggestion from somewhere which worked locally (development box) but doesn’t work on our test and production environment as they are both on HTTPS. The developer didn’t comment on that and the component he wrote was last updated in 2011. (I will find the link and update this post).