AjaxFileUpload does not work in IIS 6

I have a project (Visual Studio 2013, .Net 4.0) with a page set up using the AjaxFileUpload control from the AJAX Control Toolkit (December 2013 release). Everything works great when I run from my local PC, but when I copy it out to the web server, it no longer works (I get a red bar that just says 'error'). This is a publicly accessible web server, so there should not be any issues relating to authentication. This server is running IIS 6, and I have the web.config set up with the appropriate httpHandler according to the documentation here. We also have an intranet web server which runs IIS 7, so I copied the project there, and changed the settings in web.config for IIS 7, as specified in the documentation I linked to above, and everything works just like it did from my local PC (I used these same IIS 7 web.config settings on my local PC too).

So literally the only changes I made when going from my local PC (or the IIS 7 server) to the IIS 6 server was the web.config setting for httpHandlers, following the documentation exactly. I have not had any luck finding any further documentation on other things to try in regard to IIS 6. On the IIS 6 server I have made sure that the IIS_WPG user account has Modify access to the folder I am saving to, so that is not the issue. If anyone has any suggestions, it would be greatly appreciated, otherwise I am just going to have to just revert back to the plain old ASP.Net FileUpload control, which I would like to avoid.