Thanks for your reply. Usually we do not use plone.app.iterate, but I was following a suggestion from Plone Migration 4.3.4 to 5.0 Errors
"you need to run a upgrade-step of plone.app.iterate (working copy support) while you are still in Plone 4".
We use a lot of 3rd party and custom plugins. So our approach became focused on weeding through those libraries, eliminating the ones that are not ready for Plone 5, as well as refactoring our own plugins to work under Plone5/Python3.
So now instead of running a migration, we are building a Plone 5.2 site from scratch, and will be importing data into it.