Sessions, ActiveNetDrivers empty, crash since UE4 4.13.0

I have a simple Multiplayer project which works in 4.12.5 and 4.13 (with conversion).
I put the relevant changes in my real project, but it does not work there.

I am using How To Use Sessions In C++ as reference how to do it, because I did not find any other documentation which made any sense to me. “Look into shooter” is NOT a documentation.

I think I have all relevant changes in my real project, but instead of just working like my test project, it always crashed in
UnrealEngine.cpp line ca. 706
where there is this code:

	for (FNamedNetDriver& ActiveNetDriver : ActiveNetDrivers)
		Collector.AddReferencedObject(ActiveNetDriver.NetDriver, ReferencingObject);

and hovering tells me that ActiveNetDrivers is “Empty”.

I saw this entry in the update log:

and I think it possible that this is the reason for my problem.
Perhaps the original Project has some network vodoo which my new 4.13 project has not.

Any ideas?

I think it will not help, but here is the callstack:

 	UE4Editor-CoreUObject.dll!FGCCollector::HandleObjectReference(UObject * & Object, const UObject * ReferencingObject, const UProperty * ReferencingProperty) Line 571	C++
>	UE4Editor-Engine.dll!FWorldContext::AddReferencedObjects(FReferenceCollector & Collector, const UObject * ReferencingObject) Line 707	C++
 	UE4Editor-Engine.dll!UEngine::AddReferencedObjects(UObject * InThis, FReferenceCollector & Collector) Line 1752	C++
 	UE4Editor-CoreUObject.dll!TFastReferenceCollector<FGCReferenceProcessor,FGCCollector,FGCArrayPool,0>::ProcessObjectArray(TArray<UObject *,FDefaultAllocator> & InObjectsToSerializeArray, TRefCountPtr<FGraphEvent> & MyCompletionGraphEvent) Line 352	C++
 	UE4Editor-CoreUObject.dll!TGraphTask<TFastReferenceCollector<FGCReferenceProcessor,FGCCollector,FGCArrayPool,0>::FCollectorTask>::ExecuteTask(TArray<FBaseGraphTask *,FDefaultAllocator> & NewTasks, ENamedThreads::Type CurrentThread) Line 868	C++
 	UE4Editor-Core.dll!FTaskThreadAnyThread::ProcessTasks() Line 1255	C++
 	UE4Editor-Core.dll!FTaskThreadAnyThread::ProcessTasksUntilQuit(int QueueIndex) Line 1149	C++
 	UE4Editor-Core.dll!FTaskThreadBase::Run() Line 621	C++
 	UE4Editor-Core.dll!FRunnableThreadWin::Run() Line 74	C++
 	UE4Editor-Core.dll!FRunnableThreadWin::GuardedRun() Line 23	C++

I created a new project,
this time I put into the project the networking code first, tested it with a mini UMG, everything works.
I add the slate code, everything works.
I close the editor and the Visual Studio, rebuild everything, just a rebuild, no other changes: It crashes again.
Any idea?