Thank you for a patience and desire to help in resolving this issue.
I added “Duplicate Post” to the list of active plugins, but still did not get a described error.
We can proceed according to one of these variants:
1) you send me admin credentials for your site, I install backup plugin, create and download a copy of your site to install and work with it at my local development environment. It will be difficult to realize, if your data is too large (>100 Mb).
2) you create a copy of all files except wp-content/uploads folder and copy of database without wp_users, wp_usermeta, wp_posts, wp_postmeta tables and share them with me via DropBox or similar service. This way you will not open access to any critical or “copyrighted” information if you have any.
Of course I guarantee you a non disclosure of any information and use your files and data for the investigation purpose only.
