To reproduce, just load an invalid XML file:
            if (File.Exists("userDockState.xml"))
            {
                try
                {
                    radDock1.LoadFromXml("userDockState.xml");
                }
                catch (XmlException)
                {
                    radDock1.LoadFromXml("defaultDockState.xml");
                }
            }

Workaround:
 if (File.Exists("userDockState.xml"))
            {
                try
                {
                    radDock1.LoadFromXml("userDockState.xml");
                }
                catch (XmlException)
                {
                    radDock1.EndTransactionBlock();
                    radDock1.LoadFromXml("defaultDockState.xml");
                }
            }