WordPress-based website hosted on a Plesk server shows the following message when accessing it in the web browser:
The website is undergoing scheduled maintenance.
Sorry for the inconvenience. Come back a bit later, we will be ready soon!
On attempts to open website pages (even non-existent), there is the next error can be shown:
503 Service Unavailable
WordPress maintenance mode is enabled for the website manually, or by the WordPress upgrade task.
Disable maintenance mode for WordPress instance using WordPress Toolkit:
Go to Domains > example.com > WordPress and switch off the checkbox Maintenance mode in the card of the affected instance: