User has identified a potential crash, needs investigation: https://udn.unrealengine.com/questions/330701/crash-in-fmaterialeditornotifypostchange.html
Unknown, see https://udn.unrealengine.com/questions/330701/crash-in-fmaterialeditornotifypostchange.html for more details.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-40667 in the post.