The general reasons for this error to appear are listed in this section of the RadAjax documentation:
Can you please verify that you do handle those cases in your code implementation? Additionally, check whether commenting temporary the RadAjaxManagerProxy and wrapping the tabstrip and multipage instances inside MS UpdatePanel produces the same exception.
Another thing you can try is using the Fiddler tool
to determine the exact cause for the failure in the response parsing.
the Telerik team