501 not implented error when trying update a post in wordpress

I am getting 501 not implemented error when trying to update some of the posts in my wordpress blog.

Not Implemented

GET to /planetcoding/wp-admin/post.php not supported.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

I tried deactivating all the plugins, but the problem is still there. What should I do?

Note that the problem is not happening to all the posts, but it is happening to most.