There is nothing to change in Cocktail in order to use the RibbonWindow. Cocktail doesn't care what controls you use in your views. This behavior appears to be a quirk of the RibbonWindow. The issue is that popups in WPF are modal, so it appears that the RibbonWindow doesn't finish rendering until the modal dialog is closed.
Looks like with the RibbonWindow you can't do the login in OnViewLoaded. You'll have to do the login some other way. Maybe the user needs to click on a login button first or present the login view inside the RibbonWindow's content area instead of doing it as a popup.