I'm having a devil of a time upgrading a Plone 5.0.8 site to 5.1.4. I'm getting the dreaded "TypeError: 'ExtensionClass.ExtensionClass' object is not iterable" error.
Normally I'd see this when an add-on is uninstalled and it leaves some cruft in the database. But in this case it's a straightforward upgrade with minimal (well for me, "minimal") add-ons and nothing removed.
Any advice on how to explore this further?
Hi Sean! Can we see your buildout.cfg or at least the "non-standard" parts of it?
Sure!
There's no buildout.cfg per se since we started using Plone long before the UnifiedInstaller was a thing, so we're more-or-less using the Martin Aspelli's Über Buildout pattern, so let me just list the eggs:
jpl.mcl.site.policy
... which includes:
collective.captchacontactinfo
collective.recaptcha
eea.faceted.vocabularies
eea.facetednavigation
five.formlib == 1.0.4
pas.plugins.ldap
plone.api
plone.app.collection
plone.app.form
plone.app.imaging
plone.app.upgrade
plone.formwidget.recaptcha
Products.Archetypes
Products.ATContentTypes
Products.CMFPlacefulWorkflow
Products.CMFPlone
Products.PloneFormGen
yafowil.plone
z3c.jbot
jpl.mcl.site.knowledge
... which includes:
plone.api
plone.app.dexterity [grok relations]
plone.app.imaging
plone.app.registry
plone.app.relationfield
plone.behavior
plone.formwidget.contenttree
Products.CMFPlacefulWorkflow
Products.CMFPlone
rdflib
z3c.relationfield
jpl.mcl.site.sciencedata
... which includes:
eea.facetednavigation
jpl.mcl.site.knowledge
mysolr
plone.api
plone.app.collection
plone.app.dexterity [grok relations]
plone.app.imaging
plone.app.registry
plone.app.relationfield
plone.behavior
plone.formwidget.contenttree
Products.Archetypes
Products.ATContentTypes
Products.CMFPlacefulWorkflow
Products.CMFPlone
rdflib
z3c.relationfield
Naturally there's a known-good-versions.cfg file too. You can view it in all its hideous glory at https://github.com/MCLConsortium/mcl-site, but your eyes may burn! 
Ok that’s not really what I’d call a straightforward upgrade... a lot of addons. Do you know for a fact that all these addons work with 5.1.4? If so, you’ll maybe have to try to narrow it down by trying with fewer addons.