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Done that.

No effect.

You need to rest your pugins folder. You are still running multiple plugins.

I have reset my plugins folder.

No effect.

Not according to your site’s pages. I can references to aprils-super-functions-pack, wp-pagenavi, All-in-One Event Calendar and Jetpack.

I followed your instructions for resetting the plugins folder by FTP. What are you saying I should do?

I think you need to use the Phpmyadmin method. You have some very vampiric plugins still referenced. The Phpmyadmin emthod should drive a stake through them.

Phpmyadmin goes well beyond my level of expertise. Perhaps you would like to be given administrator access to the site for a short time? If so please contact me privately.

I’m sorry but I only offer that level of support to commercial clients.

Then you need to explain yourself. Deactivating all plugins had no effect on the symptom described at the outset. Neither did resetting the plugins by FTP.

some very vampiric plugins

Which plugins fit this pejorative description?

A plugin that cannot be deactivated fully except via the database.

The phpmyadmin that I can access via Plesk does not appear to do the job. It is designed (limited?) to setting up credentials for the database user account.

Try asking your hosts for assistance. They should be able to point towards a database management tool on the server that will allow you to make changes to a record in a database table.