1. Close the app - restart the app and try again.
2. Change to send only and try again with sync. Take a look in the webbrowser if the Changes have been received. Remove the maps and download again.
3. Move the files in Itunes and use a python-command in Arcmap to retrieve the data from the runtime-database.
We encountered this error last week, and changing the settings to Push only allowed the data to sync correctly. We then deleted and reloaded the map and everything is working again.
You are a champion, Johan!
We have seen the same issue for multiple users using the same feature on Android devices. We get the internal sync replica error, however newly created data is PUSHed back to the service. DELETE's are not.
Switching to upload only no longer throws the error and data is Pushed up. Switch back to bidirectional and Sync, no error, and data created as well as DELETED are pushed.
This doesn't explain what the root cause of the issue is.