As the old adage says, all good things must end. Beginning May 21, 2018, the Model Derivative Service will no longer support the Revit 2013 and 2014 file formats. Revit 2015 and higher will continue to be supported.
If you have an application that currently leverages 2014 and older versions of Revit, you will need to update your applications. Have questions or need help? Post it on Stack Overflow or shoot the team a note at email@example.com