(The managed object model version used to open the persistent store is incompatible with the one that was used to create the persistent store.)
We wrote back and forth but with no success. Luckily Lasse agreed to share his screen with me. In that session I had access to his Xcode project and was able to look at the issue first hand. At some point he mentioned that he is using RestKit in this project. When I first had a look at RestKit when it came out I knew that this framework was not made for me. But this is a totally different story. What I did not know about RestKit is that RestKit seems to modify the managed object model at runtime to do it's magic. This is a problem for Core Data Editor because the managed object model of your store file has to match the version of your model which is inside your app bundle.
I know that many people around the globe are using RestKit and I will try to offer some kind of workaround although I don't want to support RestKit in any way. :)
I am glad to finally know why a relatively large user base has problems with Core Data Editor. Thank you Lasse.