If the default admin account gets disabled, it will break the REST API and cause web console issues.
This is a known issue with the App Control Server listed in the Release Notes:
EP-2752: If you modify the permissions of, or disable, the "admin" user that ships with the product, The API module may no longer function correctly, causing problems when using the REST API and the console.
Although the console allows disabling the "admin" user, this should not be done.