I am getting this error in Plone 5.0.5, plone.app.iterate 3.2.0.
To reproduce: add a page, checkout page, use the rename action on the baseline to change the title (not id). Relation is now removed.
Has anyone else run into this? Within the rename action, I can isolate the problem down to the line
notify(ObjectModifiedEvent) which in turn seems to be a problem with
zc.relationfield.event.updateRelations. The process here attempts to remove relations and then add them, but
addRelations(obj, event) does not actually add a relation. At this point the inner workings of zc.relation are a little lost on me. The result is that the relation is removed, so the working copy and baseline have no connection to each other.