The Telerik.WinControls.Docking assembly is no longer part of the product, where it was in 2010. It sounds like the dock that you are using is the much older Docking Control, not the RadDock.
The current release notes state:
BREAKING CHANGE: Removed the obsolete DockingManager implementation.
The release notes can be found here
You will need to convert the old dock to the new RadDock (we also had an old one left over in our application when we upgraded and we needed to remove it and replace it with the new RadDock).
Hope this helps