--ServerApp.kernel_ws_protocol=<Unicode>
DEPRECATED. Use ZMQChannelsWebsocketConnection.kernel_ws_protocol
BaseKernelWebsocketConnection.kernel_ws_protocolUnicode
Default: None
Preferred kernel message protocol over websocket to use (default: None). If an empty string is passed, select the legacy protocol. If None, the selected protocol will depend on what the front-end supports (usually the most recent protocol supported by the back-end and the front-end).
Some time in the past few months JupyterLab installed/configured via JupyterHuab is always using the latest WebSocket prortocol. Fix will be here https://github.com/microsoft/vscode-jupyter/issues/14422 Attempted changes here https://github.com/microsoft/vscode-jupyter/pull/15897 & here https://github.com/microsoft/vscode-notebook-renderers/pull/208/files
Work around
Try overriding this value to ensure its an empty string to use the old protocol Docs here https://jupyter-server.readthedocs.io/en/latest/other/full-config.html