The credentials work and upon opening your home page in Cornerstone and then the console, I see this error:
As you can see, there's an Internal Server Error that is causing the issue. Internal Server Errors are generic though but the only thing that is sure is there's a problem with your web host because if the problem is in our product, it would also not work on our end and also all the website that runs its.
What you can do in this case is to contact your web host support and let the support staff check the server error logs so they could see what exactly is causing the issue.
I also opened your About page and Cornerstone does not completely load. I get this error in the console.
GET http://mulXXXXXdos.com/wp-content/plugins/cornerstone/assets/dist-app/js/cs-vendor.js?ver=3.5.4 net::ERR_CONNECTION_RESET 200 (OK)
It looks like this issue is only happening to previously created pages though. I tried creating a test page and Cornerstone works there.
Please also test for a plugin conflict. You can do this by deactivating all third-party plugins, and seeing if the problem remains. If it's fixed, you'll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
Since you're using many plugins, it might also be necessary to increase the memory limit. You can ask your web host to do that for your or you can do it your self by adding this code in the wp-config.php file at the root of your WordPress install.
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
Add it just before the line:
/* That’s all, stop editing! Happy blogging. */