The error you are seeing for 'DockingManager' is misleading as it still exists in 2.0. The following may help you get rid of those. Additionally, intellisense will begin to work once this is resolved.
Bad things happen with files downloaded from the internet. They have an attribute on them that leads to limited access. You have to manually remove this attribute in order for the XAML to stop fussing.
In windows explorer, right-click the file, choose properties, then click the 'Unblock' button. Do this for every individual file you are using that was downloaded from the internet. Be sure to rebuild the project to replace the blocked copy in the bin folder as well.
As a side note, I am able to still run the project whenever this is the issue. Only the XAML editor thinks that there is a problem.
I had a similar error message when I first switched to AvalonDock 2.0. This is probably a long shot, but do you have the AvalonDock.dll on a network drive? I found that once I moved the DLL to my project directory I no longer had that problem.
(I would have added this answer as a comment but I don't have rep to add comments.)