I found the Problem in my case. I had an actor that spawned other actors and kept a reference to them in the PostEditChangeChainProperty(FPropertyChangedChainEvent& PropertyChangedEvent) method. It checked if it had previously created instances of the actors and if so, deleted the old ones before spawning the new ones.
Since that method apparently gets called during level load in the editor, it created new actors during level load but somehow still kept a reference to the old (destroyed) actors.
adding this guard fixed the issue for me:
if (GIsEditor && !GIsEditorLoadingPackage && !GIsCookerLoadingPackage && !GIsInitialLoad)
hope that helps