This error can occur in various scenarios (not necessarily related to Telerik controls) however it is hard to guess the problem without having more information/code. Can you post the contents of the problematic page so that we can examine it?
Meanwhile you can also refer to this help topic where is shown how a database of a RadTreeView should look. Please node that the ParentID fields of the roots are "null" not "0".
the Telerik team
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