Webresource.axd issue

OK, this one is a little odd.

I have an rudimentary CMS type of website which does database searches, finds the appropriate header information, then provides a list of documents for that particular header. I am having an odd issue with this site which JUST popped up this week.

If I do a search using the native browser (IE11 is corporate standard), all is hunky dory. However, if I open this website inside of our WPF client application (which uses System.Windows.Controls.WebBrowser), I get this error whenever I try to use the scrolling functionality inside the grid. The attached screenshot is actually the 2nd instance of this popup, and hitting Yes to continue this second time, the whole application locks up.

Any idea why it might be locking up inside the application, but not erroring inside the native browser?

The grid is using an Infragistics WebUI UltraWebGrid if that matters at all.