Uncaught exception in ZMQStream callback

I want to run a Jupyter notebook with Julia kernel in Amazon SageMaker. The Julia 1.7.1 icon shows up in the Jupyterlab launcher and accepts the kernel on launch, but then the kernel dies immediately after launch (it never works). Log info:

[E 00:25:58.760 NotebookApp] Uncaught exception in ZMQStream callback
    Traceback (most recent call last):
      File "/home/ec2-user/anaconda3/envs/JupyterSystemEnv/lib/python3.7/site-packages/zmq/eventloop/zmqstream.py", line 431, in _run_callback
        callback(*args, **kwargs)
      File "/home/ec2-user/anaconda3/envs/JupyterSystemEnv/lib/python3.7/site-packages/notebook/services/kernels/kernelmanager.py", line 391, in record_activity
        msg = session.deserialize(fed_msg_list)
      File "/home/ec2-user/anaconda3/envs/JupyterSystemEnv/lib/python3.7/site-packages/jupyter_client/session.py", line 929, in deserialize
        raise ValueError("Invalid Signature: %r" % signature)
    ValueError: Invalid Signature: b'ec1d1093f3b6505658469b860c203f696bc39cf8fcea1672cb55802fc57592eef57b8db0b5cb603d1bcada6f41060f0819f6002a7a31f309fbaa1a701cc13f5b'

There are some posts from 2018 recommending updating tornado and ipykernel, but 4 years later this should hardly apply (and I tried it, and it didn’t work). Any suggestions? What could be going wrong here?