The problem

After updating your WordPress and/or WP Maintenance Mode plugin your site is running normally for a period of time eg between 6 and 18 hours. Then at some random time all that any visitor is able to see is an Internal Server Error message. To anyone trying to get to your site, your site appears to be broken. For more information open the section on Server Side Errors in my article, Where to find Apache response codes and how to determine their meaning.