I was trying to merge a migrated, and already closed question, but the moderator menu didn't allow me to merge the questions.
As the merge is from the viewed question to the question for which I would enter the URL, or the post ID, why isn't merging possible in this case?
When I tried from http://drupal.stackexchange.com/admin/merge-questions the merge was done without problems.
Update: It happened again with another question, which was migrated from Stack Overflow, and closed as duplicate.
I think the problem is trying to merge a migrated question with an existing question, as I don't have the same problem with questions that are not migrated. I know there have been changes to allow automatic migration rejections, but merging should be allowed, even if the question comes from another site.