I do not believe this issue is related to the RadToolTipManager, but it is rather a matter of permissions and access levels in your application. The user control class should be public and should reside in the same namespace as the page that tries to access it (i.e. is simply should be accessible). In our example it is also in the same directory, so if yours isn't please make sure that the path is correct and accessible as well.
Generally speaking - once you are able to load it inside a simple asp Panel it should work for the RadToolTipManager as well.
the Telerik team
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