I have solved the issue by recreating my control forms from ground up new. The problem maybe occurs due to mixture of used diagram assemblies after updating from v14 to v15. Rebuild does not solve the issue.
The deserialization statement then had a problem where all tool names within the NDiagramView.Controller.Tool collection are unnamed (Name property = null). Every time when I opened the VS designer the Form.resx was changed and after that the tool names were invalid.
this.nDrawingView.DesignTimeState = ((Nevron.Diagram.NBinaryState)(resources.GetObject("nDrawingView.DesignTimeState"))); By the way.
The Nevron support was really fast and highly committed to solve the issue! Thank you, therefore.