It looks like this is some fallout from CL 19405683. If we change a variable from vector to transform, it'll pass this check:
We shouldn't use "UseSerializeItem". We'll serialize the vector data as a transform, which leads to incorrect data and a subsequent crash, since we slurped up the binary stream incorrectly.
A sample project has also been provided.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-168261 in the post.