After updating to the latest version of Xamarin.Forms you may get an error related with Android project.
If you do so, you may get this error because Xamarin.Forms is targeting specific version of Android packages and by doing an update to all packages, a more recent versions of Xamarin.Android.Support is installed. That may not be compatible with Xamarin.Forms version you are installing.
To make a quick fix on this, just remove all Nuget packages from the Android project and add again Xamarin.Forms. Doing so will add the proper dependencies and the error will be fixed.
Update all is not always a good choice I guess.