Hi there,
Cornerstone works correctly with the normal recommendation of the WordPress. For more information:
https://codex.wordpress.org/Changing_File_Permissions
According to the article above the recommended file permissions for files is 644 and for the folders is 755. Advise your hosting service provider and check if they may need other permissions for their settings.
Also, I suggest that you enable the WordPress to debug mode so instead of the error 500 which my colleague gave the screenshot, you will see the actual error:
https://codex.wordpress.org/Debugging_in_WordPress
I also suggest that you ask your hosting service provider to give you the error log so you can check the case.
Lastly kindly make sure that the cornerstone URLs which in your case is www.picotera.co/x
and all it’s sub URLs will be ruled out of your hosting service provider mod_security
settings. This is something that they can change on their side and you probably do not have access.
Thank you.