One of my blog had this issue appearing a bit out of the blue. Clicking update the post would just redirect me to a blog page 404 with the url wp-admin/post.php
How to solve the WordPress post update returning a 404 page
Looking online people are talking about permalinks, changing the .htaccess.
Nothing was changed there but I gave it a go. No luck, it’s not permission or anything as my links were working fine on the front-end. It was the 404 of my blog theme I was seeing when clicking the WordPress update button.
The issue was the WordPress plugin named “Classic Editor”, Enables the WordPress Classic Editor and the old-style Edit Post screen with TinyMCE, Meta Boxes, etc. Supports the older plugins that extend this screen.
No biggies on that specific blog, I can use Gutenberg editor so that was a simple fix.
In case you have that same issue, check if this specific plugin or another can be conflicting.