Hello There,
Thanks for providing the login credentials. I was able to logged in and disable Yoast SEO and All in One SEO plugin. I was able to figure out the cause of the issue. It turns out that when Cornerstone is loaded, the PHP memory limit is exhausted. By default, WordPress will attempt to increase memory allocated to PHP to 40MB (code is at the beginning of /wp-includes/default-constants.php) for single site and 64MB for multisite, so the setting in wp-config.php should reflect something higher than 40MB or 64MB depending on your setup. Upon installing All in One SEO, it automatically changes your PHP memory limit and even have the settings to change it up to 256MB. This is why, when this plugin is active, you can edit in Cornerstone.
At the moment, this error is showing up in your dashboard because I have disabled the plugin. The only way to resolve this is to login to your ftp and change the PHP memory limit. Please provide us your ftp access so that we can help you fix this issue.
Hope to get a response anytime soon.