Hi Mike,
Can you give us some more details about your data? Ex. any turn feature classes? using the Global Turn Delays? Multimodal? Is the network dataset status Built? Any other little things you can think of? Has the directions.xml or resources.xml files from <Arcgis Install Dir>\NetworkAnalyst\Directions> been modified?
When you say that routing service on Server fails, how does it fail? Does it generate an error message? If so, what is the exact wording of the error?
When you upgraded to 10.5, did you upgrade ArcGIS for Desktop and ArcGIS Enterprise (formerly ArcGIS for Server) or just Desktop? How was the upgrade done?
The answers to those questions should hopefully give us a starting place to figure out what is going on with the directions.
-Rachel