An actor channel's InPartialBunch won't be deleted and cleared until a new partial bunch is received. This can result in a situation where memreport counts the memory for an old InPartialBunch and its next bunches, but the Next pointer is invalid while not being null.
When running memreport while an actor channel has an InPartialBunch (and that InPartialBunch has a seemingly valid Next pointer), a read access violation can occur during FBitReader::CountMemory for InPartialBunch.
See linked UDN for more info.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-169654 in the post.