Hi all
I ran into the same issue when moving to 5.6 and determined it was because the SteamNetDriver has been moved. SteamNetDriver now exists in the SocketSubsystemSteamIP module as opposed to the original location in OnlineSubsystemSteam.
The fixed ini config is as follow:
[/Script/Engine.GameEngine]
!NetDriverDefinitions=ClearArray
+NetDriverDefinitions=(DefName="GameNetDriver",DriverClassName="SocketSubsystemSteamIP.SteamNetDriver",DriverClassNameFallback="OnlineSubsystemUtils.IpNetDriver")
However, I suggest moving to the new SteamSockets (as suggested in this thread) as it will probably be more future-proof.
Cheers!