Thanks for the solution, it's working fine now for the radcombobox!
But i've got a second problem when adding standard button and checkbox controls to the usercontrol thats loading asynchronously.
I get an error with the message that these controls has to be inside the form-element with a runat="server" property.
The issue is there is no form element in the page created by the RenderView method and there seems to be no controls like radbutton or radcheckbox to use a RegisterWithScriptManager property.
Should I try to add the form element by own code and remove it later from the output, or do you have an easier solution?
Thanks for your assistance,