Don't know if this is a Server issue or a ArcGIS Runtime SDK for .NET (Quartz) issue. Also I am downloading from a versioned database configuration
When trying to create a Offline DB from a service on a Federated ArcGIS Server it will fail. It does not give a whole lot of information in the server log to indicate that anything went wrong. I have done this connecting using Credentials of the item owner (which fails and the server log shows a credentials issue). I have also done with a administrator which still fails but does not show a credentials error. I know the credentials are valid because I can connect to and get PortalItems using the same credentials.
If I try the same operation on an ArcGIS Installation that is not federated but instead just uses AGS security everything works as expected. The operation creates the new version and the replica database is downloaded to the device running the application.
Something else interesting is that if I run the createReplica command through a web browser passing the same parameters as used by the Runtime application (except setting async=false) it will work - But ONLY if the version has been created in the database. It fails if a properly named version does not exist, so it does not create a version as it should. Also if running through the browser and setting async = true if will fail
Solution is same as here https://community.esri.com/thread/182706-map-failed-to-download-thread-2 . Basically, in order to download a offline database you need to publish the service from ArcMap. Even if you go into Server Manager and add the Sync and Extract capabilities those capabilities do not work and you cannot download.