The issue might be caused by the fact that the RadDock control implements the INamingContainer interface and its ContentContainer also implements the INamingContainer interface. This means that the UniqueID and the ID of each control inside the dock will be different, and the control with ID="ContentPane" cannot be found simply by searching the default naming container.
To show this, please place the controls with fixed IDs inside any control implementing INamingContainer interface, like asp:Repeater control, bind the repeater in the code behind to a dummy data source, and you should experience the same behavior.
I cannot recommend a solution to this problem, because I am not familiar with the DNN framework. Please post the question in DNN forums. Someone should have had similar issues with naming containers.
the Telerik team