We are not sure what can be causing the problem, and it has never been reported before and have no suggestions. You could try to resolve this error in the same way as for the Response.Redirect method.
However, if the issue persist it will be best if you open a formal support ticket and send us a simple working project with reproduced this erroneous behavior.
Additionally, for more information about RadAjaxManager Redirect()
method you could review this helps topic:
Redirecting to another page
the Telerik team