Hi there @witali,
Thanks for writing in!
I've taken a look at this and you're getting a 500 internal server error when trying to edit. A couple of things:
1: Your .htaccess, the WordPress rules should be at the top with the redirects under that. Right now, you have redirects, WordPress, redirects. Cleaning that up could have an impact.
2: A internal server error, usually points to an issue with a plugin conflict or a server side issue. As it was working before, it'd lead me to believe the issue is a plugin conflict.
Have you gone through and tested for a plugin conflict? If not trying deactivating everything apart from Cornerstone, and then try editing a page. If it works, re-enable plugins one by one until it no longer works.