Magento 2 site throws an exception instead of the “Service Temporarily Unavailable” page in the maintenance mode.
Reason: The reason behind throwing this exception is that the current Magento site is in the developer mode. As we know that in developer mode, errors/exceptions are directly displayed in the browser to the end-users, this exception “Exception #0 (Exception): Unable to proceed: the maintenance mode is enabled” is displayed directly on the browser.
Solution: To display the “Service Temporarily Unavailable” page in the maintenance mode, we have to change the application’s mode to production. In the production mode, exceptions are not directly displayed in the browser to the end-users, logged in the files instead. Thus, we can view the “Service Temporarily Unavailable” page now in the maintenance mode.
