It seems that there is the duplicate of the plugin somehow causing the problem. Kindly go to wp-content/plugins/ and delete the cornerstone folder completely from there.
Then make sure that the theme is updated to the latest version. After that go to X > Overview. Wait for few seconds and go to your Wordpress Dashboard, you should see a message that Cornerstone is installed automatically.
Needless to say to have that automatic installation you need to make sure your installed instance is validated.
Hope it helps.