Install Wizard indicates that importing data is a valid step in upgrading. This is not the case.

In the setup wizard in Load Content, you will see this sentence under 'Restore from Backup':

Use data from a previous installation of Confluence. If you are upgrading or replicating Confluence you will probably want to select this option.

This is incorrect - if you are upgrading Confluence you should never see the setup wizard. If you have reached this point during an upgrade, something has gone wrong.

This sentence should read something more along the lines of:

Use data from another installation of Confluence. If you are migrating to another database or replicating Confluence you will probably want to select this option.

Workaround

Please see the instructions for Upgrading Confluence for the correct upgrade procedure.