My Nexus 2.14.4 has 2 group repositories containing only Staging repositories. Going through the upgrade steps in Nexus 3.3.0, I can select those groups, but the Staging Repos are not able to be selected. Also, I have an empty group, which is not able to be selected. See Screenshot.
During the Synchronizing step, it processes 19 of the 21 repositories, and the "Process Changes" completes and waits for changes on the Nexus 2 side, but the last 2 Repositories never complete "Scanning". The message in the log is shown below as well as screenshot.
Rerunning the upgrade without selecting these two groups is successful.