We have checked your site and don’t see problem with posts. Have you sorted out or still have a problem? Do you have problem on frontend or backend? If your problem is related to backend can you, please, provide me temporary Dashboard and FTP access to your site?
From your description, it is server resource shortage or server settings.
If your server is using mod_security or FastCGI handler for PHP you need to contact with your hosting provider and ask to increase the maximum body length value.
I have added code in your .htaccess file to implement this but it would be better if the provider changes it in the server settings.