I am creating this post for Andrew, per this article:
I worked with Andrew to ultimately create a Discovery Case that only includes the messages that were previously "exported" to a Smart Tag by the Case Review App. So now there is a standalone Discovery Case which includes a single Saved Search based on a single Smart Tag.
This case appears in the eDiscovery Export app, and gives Andrew the option to export the case as it should, but the app is returning "An unknown error occurred."
Posting here so that the beta team can provide next steps for troubleshooting.