Found the issue. Thank you @oberstet for commenting (see above). While just setting the client realm to suggested default realm1 did not work, that lead me towards the solution.
The short and simple answer is that the Python WAMP server should always explicitly set the realm. This applies at least when the server implements a component that subclasses wamp.ApplicationSession (many or all the examples at the autobahn website).
In other words, in the server there has to be an onConnect function that joins a realm. Without it,
- a similarly constructed Python client (that does not join a realm), will work
- no (Python or Javascript) client that sets a realm will work
Some speculation beyond the solution: If one can build a Javascript client that works without setting a realm (example I used requires explicit realm), perhaps such a client would work as well. Perhaps the Python WAMP server code is buggy in its setting of default realm. Or maybe it's just that the default has been changed. I don't know.