@agitator is this really necessary? The plone.app.contenttypes folder should also implement plone.folder.interfaces.IFolder.
I also had never problems using this package under Plone 5.
See:
>>> from plone.app.contenttypes.interfaces import IFolder
>>> from plone.folder.interfaces import IFolder
>>> from plone.app.contenttypes.interfaces import IFolder as ctf
>>> from plone.folder.interfaces import IFolder as pff
>>> ctf.providedBy(app.Plone.events)
True
>>> pff.providedBy(app.Plone.events)
True
@agitator is this really necessary? The plone.app.contenttypes folder should also implement plone.folder.interfaces.IFolder. I also had never problems using this package under Plone 5. See: