Hopedully saves someone some frustration - using UE4 4.21.1 if you start with the MultiUserViewer template map (everythign working fine so far), create a new map and import a Datasmith export everything will seem fine until you hit Play… then everything will be black. It wont matter your lighting setup or if you built lights or anything - totally black viewport and no error messages.You can press F2 and see that everything is there and you can walk around, but switch back to Lit viewport and its totally black.
The quick fix is to go into Edit->Project Settings->Maps and Modes and change Game Instance Class to Game Instance instead of BP_MultiUserGameInstance
I have not looked yet at what needs to be set to allow using BP_MultiUserGameInstance on user created maps. All the other things like pawn and player controller work with custom maps, just not the provided game instance. This quick fix will probably break multi user for your custom map and AFAIK there is no way to do something like have a map set the game isntance in on begin play or anything like that (there is no SetGameInstance node in BP and the C++ method UWorld::SetGameInstance looks kind of shady to me - is it something that can be safely used at runtime?)
I’ll post a more “correct” fix when I get a minute - but this at least gets things basically working