There is a way to add known types, but since this was working in DF2010 it's much more likely that the method call isn't quite right.
This signature has always been a bit ugly. Before, you needed to supply a callback and user state, even if null, and then your RPC method arguments. So something like this:
manager.InvokeServerMethodAsync("SomeServerType", "SomeMethod", callback, userState, arg1, arg2, ... argN)
Now you can instead do:
await manager.InvokeServerMethodAsync("SomeServerType", "SomeMethod", arg1, arg2, ... argN);
If you're supplying a CancellationToken if would come before your arguments. If you're also referencing the DevForce 2012 Compatibility pack that might confuse things further.