Well, making some changes I find out a solution, but not the cause.
I had three entities in my Core Data stack (Podcast, PodcastDownload and PodcastPlayback). The main entity is Podcast and PodcastDownload was used to save information about the download, as I explained in my question.
What I did was take the attributes out from PodcastDownload and put them in the Podcast entity. And now seems to work as it should.
I don't know the exact cause of the error, but I think it was related with threading (sorry, I'm still a little noob with objective-c).
Thanks for all the help.