If you get a 502 error when you try to edit with Elementor you have to do two things:
Go to Elementor > Settings > Advanced, and under 『Editor Loader』, enable 『Switch front-end editor loader method』.
This setting changes the Elementor editor loader method.
You then may need to increase a particular Apache service limit. To do that, add the following lines inside the website』s .htaccess file:
SubstituteMaxLineLength 10M
You can edit that file via SSH, FTP, or the File Manager tool in cPanel.