Cannot update old posts in WordPress 4.0

I’ve looked at a single post to make it simpler.
Theme Plugins Error count
2014 No plugins 5
2014 All plugins 5
2012 No plugins 15
2012 All plugins 15
Publish All plugins 15
Child All plugins 16

Iv’e had a a look at the 2014 errors and they are:
1) use of <p align=”center”> which is obsolete.
2) The frameborder attribute on the iframe element is obsolete. Use CSS instead.
3) this one looks like a possible cause:

Bad value attachment wp-att-2178 for attribute rel on element a: The string wp-att-2178 is not a registered keyword.
…5-50-45/” rel=”attachment wp-att-2178″><img class=”alignright wp-image-2178 si…
Syntax of list of link-type keywords:
A whitespace-separated list of link types, with no duplicate keywords in the list. Each link type must be listed as allowed on and <area> in the HTML specification, or must be listed as allowed on and <area> on the Microformats wiki, or must be an absolute URL. You can register link types on the Microformats wiki yourself.

I don’t think the problem is related to the themes. I think I will back the whole lot up and start a new install of WordPress and see if I can make posts containing images with that.

It must be related to the database – I’ve got a completely Fresh install of WordPress using the old database and it still does it.

I’ve done a completely new install. Wiped all content and the database.

I still get the same problem with WordPress 4.0!
If I try to add an image to a post then the result is:

Not Acceptable

An appropriate representation of the requested resource /wp-admin/post.php could not be found on this server.

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

Searching for that lead to this where they had exactly the same problem as me!
https://www.utopiamechanicus.com/article/406-error-wordpress/

I believe this is the answer”

MOD_SECURITY is a filter that tries to prevent ‘bad’ words from getting into the forms via POST (or GET). In this case, it thought the <img> (image) tag’s src= entry URL with “https://” in it might be an attempt at hacking, and likely /%category%/%postname%/ an attempt to hide data (the % is used for encoding).

“By just turning off the POST filtering, the rest of MOD_SECURITY remains active.

Oddly enough, for my client, none of these worked. I figured it must be something else (like some server filtering, perhaps). But since I had explored all the options, I was certain that WordPress was doing nothing, and it must be a server setting. So I asked my client to contact support, mentioning that certain keywords were being filtered.

The result? THEY turned off MOD_SECURITY, and his site was as good as new!

That fix worked. RESOLVED!