Do static SceneIDs actually work?

I built a minimal case.
It has the same binary for client and server.
It has one Level.
It has one object with one networkView with one static Scene-allocated viewID.

In OnPlayerConnected I send a simple RPC call with a string to the client as a server message to acknowledge the connection.

The client gives the folling error message:

Could't invoke RPC function 'serverMessage' because the networkView 'SceneID: 1 Level Prefix: 0' doesn't exist

Which is strange, because there is only one level. And I never set any level prefixes. And the SceneID are both 1 on client and server, as my logging shows. The server claims it as its own, the client doesn’t. The group is the same 0.
This happens regardless of whether server or client run in the editor, or both run standalone. It also happens if I explicitely set the LevelPrefix to 0 on connect.

Is there any way to establish a server/client communication without using Network.Instantiate?

Because statically allocated IDs in the level/scene apparently don’t work.
And if you don’t have a synced viewID on both sides already, you also can’t send any ID allocated with Network.AllocateViewID, either from server to client or from client to server.

What channel does Network.Instantiate use if there are no working viewIDs yet?

Am I absolutely required to use Network.instantiate to have any chance of establishing communication?

If I have all my game object in the scene already, do I really have to delete them and recreate with Network.instantiate or reload the whole level?

I figured it out. It wasn’t the net code at all. It was the editor that scrambled the IDs without showing anything about it.

When you just get started, and only have crappy documentation, this is about the last thing you think of.

But the editor doesn’t only not show that anything is wrong, it also prevents you from editing the SceneIDs. I had to manually remove all network views and recreate them.
And be careful not to use Ctrl-D to duplicate any objects with network views. It duplicates the ID too (which you then can’t edit).

Which is doubly stupid. First they prevent you from editing the numbers yourself, because they think you just shoot yourself in the foot. And then they do the footshooting for you.