Migration of a subscription fails in Plesk Migrator: Unable to restore Plesk extension 'wp-toolkit' for domain 'example.com' on target Plesk server

Symptoms

Migration of a subscription fails with the following error message in Plesk Migrator:

PLESK_ERROR: Unable to restore Plesk extension 'wp-toolkit' for domain 'example.com' on target Plesk server; domain data of this extension will not be deployed. You need to configure Plesk extension 'wp-toolkit' for domain 'example.com' manually on target Plesk server after migration

Cause

WordPress installation could not be properly registered in WP Toolkit due to some issue. 

This is a bad error reporting bug in WP Toolkit with ID PMT-5640.

Resolution

The real error message, that explains why the WordPress installation could not be registered, is hiding in the migration log with debug messages checked:

1. Log in to Plesk.
2. Go to Tools & Settings > Migration & Transfer Manager > 203.0.113.2.
3. Switch to Log tab and check Show debug messages.
4. Expand the error logs with red exclamation mark to see the real error message.