Hey Ivan,
Please try uninstalling Cornerstone and then go to WP Admin Menu > X and click on the link to install Cornerstone again.

Please also have your web host prevent error messages from displaying. That could be done by changing this line:
define( 'WP_DEBUG', true );
in your wp-config.php in the root of your WordPress install to this:
// Enable WP_DEBUG mode
define( 'WP_DEBUG', true );
// Enable Debug logging to the /wp-content/debug.log file
define( 'WP_DEBUG_LOG', true );
// Disable display of errors and warnings
define( 'WP_DEBUG_DISPLAY', false );
@ini_set( 'display_errors', 0 );
That will disable error display while still logging the errors in a debug.log file.
Hope that helps.