Hello,
Problem is not related to our theme because the same issue appears even with default WP theme if you enable it and check.
It might be an issue with WordPress core or a conflict with it and the server setup. A quick workaround would be to use child theme style.css file for all the CSS changes and edit the file via FTP.
Read this post, maybe it will help you to find our how to solve the problem with editing via Dashboard https://x10hosting.com/community/threads/wordpress-theme-editor-redirects-to-an-error-404-page-on-my-blog-urgent.200038/
Regards