Cannot open the WP-Toolkit Plugin in WordPress: Plugin Access token not found

Symptoms

• Cannot open the WP-Toolkit plugin from WordPress Dashboard. The following error appears in the screen:

  

• The following error is logged in /var/log/plesk/panel.log:

PLESK_INFO: ERR [panel] Access to API is disabled by admin access policy for ::1

• The following error shows in domain's access logs:

PLESK_INFO: FastCGI sent in stderr: "PHP message: Failed to get apiUrl from token data" while reading response header from upstream

• IP Address Restrictions are enabled for administrative logins.

Cause

When IP Address Restrictions are configured for Administrator access, Plesk enforces that policy against API requests, including those from ::1. 
Since ::1 is not explicitly included in the allowed list, the API call is denied and the plugin fails to load.

Resolution

Add the address ::1 to the network list:

1. Login to Plesk
2. Navigate to Tools & Settings > IP Access Restriction Management
3. Add ::1 to the network list

For the existing WordPress installation with the error, it is necessary to remove the WP-Toolkit plugin and then reinstall it. 
New WordPress installations will work correctly.

Note: ::1 is the loopback or localhost address. The IPv6 equivalent of 127.0.0.1. It is safe to add.