Hi Mike,
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.
Sincerely yours,
Simon
the Telerik team