Why does my Resume Upgrade Always Fail?

The Problem

After upgrading to Popup Maker v1.4+, and running the DB upgrade you are getting an error Message on admin pages:
"Popup Maker needs to complete a database upgrade that was previously started, click here to resume the upgrade."
When you try to resume the upgrade it shows:
"Upgrade failed, please try again.".

The Underlying Issue

Generally the update has finished but never cleared the stored value that is used to resume updates.

The Solution

  1. Add this line of code: delete_option( 'pum_doing_upgrade' ); 

either to the code editor of the 'My Custom Functions' plugin, or within the functions.php file of your site theme (child or parent theme). 

  2. Refresh your browser a few times until it disappears.

  3. Edit your functions.php file and remove that line.