The Case Gold exporter from Fluent version 17.2 contains a bug, that will make the geometry of datasets containing polyhedral meshes not correct. This results in case gold dataset that can't be loaded in EnSight.
The Fluent development team has already identified and fixed the issue. The next release of Fluent will contain the correct Case Gold exporter.
If you are running Fluent version 17.2, please consider upgrading your version of Fluent to get the fix. Failing that, you can export the dataset in Fluent format.