First, try this to see the emails: Tools | Options | Environment | General tab | remove the tick by Enable Conversation Threading

That should at least get you seeing the emails.

Second, might assist you to open a service request and get the latest FTF build of the 18.0.2 client.

Just be aware that your freshly migrated GroupWise system is outdated as GroupWise 18.1, a major version release, was shipped on 31 October 2018 - just letting you know.