Testing a migration of one of our Plone 5.0.5 sites to 5.1.5, I get this error when adding a reference in the related items field.
Traceback (innermost last): Module ZPublisher.Publish, line 146, in publish Module Zope2.App.startup, line 303, in commit Module transaction._manager, line 131, in commit Module transaction._transaction, line 310, in commit Module transaction._transaction, line 301, in commit Module transaction._transaction, line 446, in _commitResources Module transaction._transaction, line 420, in _commitResources Module ZODB.Connection, line 498, in commit Module ZODB.Connection, line 547, in _commit Module ZODB.Connection, line 579, in _store_objects Module ZODB.serialize, line 419, in serialize Module ZODB.serialize, line 428, in _dump PicklingError: Can't pickle <class 'grokcore.component.interfaces.IContext'>: import of module grokcore.component.interfaces failed
five.grok was part of our 5.0 environment (I think still included with Plone?) but it is gone from 5.1. None of my code uses it anymore. I would guess some context objects are still providing that interface? I don't know how that even works without a class there - removing zope component architecture is a PITA