User Moo__ in the #plone channel at Freenode alerted me to the solution. What happens is this:
- You uninstall a product.
- Plone detects this, and to prevent malfunctions, any of the content types in the product get registered in a secret exclusion list. Whenever the navigation is shown, this list is consulted so as to not make Plone fail because the type of the uninstalled objects can't be found.
- You install the product again.
- Items remain in this secret list, and do not get removed no matter how hard you tinker with the Navigation control panel.
The solution is to remove the excluded content types in the
portal_properties folder of your Zope Management Interface:
- Open your ZMI.
- Browse to
portal_propertieswithin your Plone site.
- Go to the Properties tab and look for the list
- Remove the content types that got added there.
That's it! Now, why isn't this documented somewhere?