Can't target Android 6.0

I have a Xamarin Forms app with a Droid project. I have Android 6.0 (API 23) installed via SDK Manager. But Xamarin Studio only shows up to 5.1 in Options / General / Target Framework.

I reinstalled 6.0 via SDK manager but no luck. Any ideas?

Edit: The issue appears to be that my Xamarin Studio won't update. Checking for updates throws "Error retrieving update information"