The latest Xamarin update for mac has several severe issues. The mac 'apple' menu is duplicated, and the monogame plugin is not recognized anymore. To me, this makes Xamarin unusable. How can I uninstall the latest update?
Your MonoGame Addin is failing, but I know it works in the latest release as I am using it...
Try this:
${HOME}/Library/Application Support/XamarinStudio-5.0/LocalInstall/Addins/MonoDevelop.MonoGame.3.4.0.455
If it still fails with the same error message, it is another of your locally installed Addins that is loading before MonoGame that is causing the cyclic failure. Remove them one by one (or all) to test which one is the cause.