I suggest you replace the first argument of the __doPostBack
function with an empty string ("") if you do not need it on the server. In case you need it, I suggest you replace it with another identifier for the TreeView, different from its ID, like 'rt1' for example.
The problem is caused by this first argument: the TreeView on the page recognizes this postback as directed to it (from the first argument) and tries to parse the second argument which produces the error.
the Telerik team