UNetDriver::DestroyedStartupOrDormantActors maintains a list of deleted startup (and dormant) actors.
This is used to notify clients that have joined the server of Actors that need to be destroyed or marked dormant to synchronize state with the server.
However, these entries are stored as GUIDs that can be used against the PackageMap (not actor pointers), and are not removed when the owning sublevel is unloaded.
Since the package map / guids aren't necessarily cleared, this can leave dangling references to the actor.
Client connects to the server with no issues.
Server crashes when client tries to connect.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-47192 in the post.