switching from NetworkManager to LobbyNetworkManager: NullReferenceException

Hi,
I'm making a multiplayer game, and I started by using the NetworkManager before hearing about the NetworkLobbyManager.
The game works fine on LAN with the NetworkManager, and I'm trying to switch to LobbyNetworkManager, but I get a "NullReferenceException: Object reference not set to an instance of an object" error on a script, and my main gameplay feature (which uses Rpc and Cmd functions) stops working.
The weird thing is, if I disable the LobbyNetworkManager and re-enable the NetworkManager, it works again (so it is not that I accidentally disabled something).

Is there something special you have to do to use Rpc and Cmd functions on the LobbyNetworkManager?

Thanks for your help