The Cornerstone preview URL will use what is being set in Settings > General > Site URL. If this url is not the same as your dashboard url, then the issue will occur.
I already tried changing the url and it does not saved. This usually happens in some staging sites wherein you cannot change the site url via settings. What you can do is to log in to your ftp, browse to the folder of the staging site and edit the wp-config.php file and add this:
Please do not forget to replace
example.com with your actual staging site url.
Hope this helps. Kindly let us know.