AjaxFileUpload causes javascript error with authorization

Using AjaxFileUpload in WebForms targetting .NET 4.5.1, I get a javascript error only under a certain condition. This is the error:

throw "error raising upload complete event and start new upload"

And the condition:

Error only occurs when I include a web.config authorization element somewhere in the page's directory tree: deny users="?"

If I comment out the condition to restrict to authenticated users, AjaxFileUpload works successfully. If I apply this as a single condition in the authorization element (deny users), then the error is thrown.