ObjectDock's Start Menu button seems to send the Winkey keystroke and whatever is set to open the Windows key in Classic Start Menu settings will open.
You could try another workaround to open the menu. Right click on dock background -> Settings -> Add -> Shortcut -> Blank shortcut. In the 'Link' field, specify: C:\Program Files\Classic Shell\Classic Start menu.exe and in the 'Arguments' field specify: -toggle. The -toggle parameter opens Classic Start Menu. You can rename and change the icon to the one used for the Start button. Does that work?
