To troubleshoot this error, we need a local reproduction. Is it possible for you to share a copy of the 'HTMLTestpage.htm' file?
As an alternative you can record a Fiddler trace
of that page. With that trace we'll be able to "simulate" your app in order to reproduce the problem.
If you regard that information as sensitive, you can attach it to a support ticket which is confidential, unlike this forum thread.
All the best,
the Telerik team