In my French class we had one exercise to choose the right form of the past participle and even the teacher (a French native) wasn't sure what the correct answer was. It's kind of an awkwardly structured phrase, which doesn't help either:
Des problèmes, ils s'en sont créé / créés toute leur vie !
The book says the correct answer is "créés", but in the rules we got from a different text they say there should be no agreement when "en" is the COD of the verb. However, that section is talking about verbs that are conjugated with "avoir" and offers no specific rules for "être" and "en".
I would like to know from someone knowledgeable enough in the subtleties of French conjugation which one they consider correct and if you have an authoritative source to cite, that'd be perfect. Thanks!