1. CKeditor is missing / not installed.

Make sure the ckeditor is installed if the page is supposed to have any HTML editor.

2. Issue could be related to server modsecurity

In case if you face error 403 or 404, it could be because of the server modsecurity settings. In this case, you will need to contact your server support.

Reference Material:
https://www.namecheap.com/support/knowledgebase/article.aspx/9542/22/what-is-modsecurity-and-why-do-we-need-it