Hi DI Johannes Leitner.
Did u get the solution of your problem?
I am having the same problem and I am able to reproduce the error. Here is my scenario:
I've a Menu Control where I am using Rad Tree View and I am using it on Master Page which in turn is used on all other pages. My Project is using almost all of Telerik Controls. There is one specific page where I've a simple data grid( not Rad Grid) . If i do any operation on that grid, and try to access the menu, it gives "Sys.ArgumentTypeException: The object with the type Telerik.Web.UI.RadTreeView can not be converted to the Type Telerik.Web.UI.RadTreeView".
Menu Control is working fine on all other pages, I am thinking to replace the data grid with Rad Grid, but it is not the solution of the problem.
any ideas ?
btw i m using version: v2009.1.311.20
Thanks and Regards.