Error: "The URL <URL> cannot be added since it returned a 5xx error code, this indicates an internal server error on your site. Please make sure it is accessible and not in maintenance mode."

In order for us to properly start monitoring your application, its response when you first add it must not be a 5xx HTTP status code.

When you put your site in maintenance mode, it will also result in a 5xx HTTP status code and thus will trigger this error.