Do you have login page in this sites that browsers can use auto-complete to fill username/passwords?
If the browser has stored some of the hidden fields and now it send them, and we expect different format, and parsing error will happen.
If some of your sites is public, can you share a link with us?
All the best,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed