I've lost track of the different servers involved here! @sowe1 is yours the same as @ehbossa's? And that's the one that worked after a configuration change?
This looks like the same issue to me and the same as Media file generate a bad path as well. You can see from the 500 error that Enketo is unable to request the CSV from Central because of a private IP address issue.
The possibility of this problem has been removed in recent Central versions. v1.3 is now getting pretty old and I would strongly recommend upgrading as soon as you can. You'll want to make sure to have a backup and plan for a maintenance window to go through the several changes that have happened between that version and current ones.