'Creating default object' error, and post edits not being saved

A client started seeing this error recently in 3.5.1, so I upgraded the site to 3.9 (plus all plugins, etc.), but still no joy.

First sign of a problem was ALL pages had been set to ‘Include this page in lists of pages’ on ‘Exclude Pages’ plugin (when only a few should be displayed on main nav), and ALL pages were therefore displayed on website’s main nav bar. Then it became apparent that any attempt to edit/create a post or page firstly displayed the following error at the top of the edit screen:

“Warning: Creating default object from empty value in /homepages/8/…/post.php on line 569”

Then when saving the edit, nothing was saved and we were redirected to the Posts listing admin screen.

Another ‘anomaly’ is that, although WP dashboard displays the WP version correctly (3.9), on the Updates screen it is shown as ‘Version -‘ with a ‘Download -‘, and the ‘Re-install Now’ button doesn’t work – it just opens up a blank screen!

We have re-installed 3.9 manually (deleting all wp-admin and wp-includes files, and all root files).

We have re-activated Twenty Eleven/Twelve/Fourteen themes.

We have deactivated all plugins.

With all changes above we still get the errors.

Have searched extensively for solutions to the ‘Creating default object’ error, but solutions seem to me to be concerned with hiding the error rather than fixing the cause. I’m sure the error is not in the post.php file, but indicative of an error elsewhere.

Any ideas of what else to try would be greatly appreciated.

Many thanks,

Andy