title: Impossible to select kernel in visual studio code, findKernel crashed : 503 Service Unavailable
body: <p>I am using Visual Studio Code with the Jupyter extension (ms-toolsai.jupyter) version 2021.8.1054968649, I am connecting to a container over ssh (so I use both Remote - Container and Remote - SSH). For various reasons I cannot use the default local version of the Jupyter, I have to run on an existing Jupyter notebook.
  It used to work, but now the extension cannot find any kernel (should be an option on the far right) :
  <a href="https://i.stack.imgur.com/EPIHo.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/EPIHo.png" alt="Kernel selection" /></a></p>
  <p>When I try to start the first cell it proposes me to select a kernel, but there is none :
  <a href="https://i.stack.imgur.com/x9NEj.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/x9NEj.png" alt="Kernel selection 2" /></a></p>
  <p>After looking into the log it seems that I get an error 503</p>
  <pre><code>Error 2021-07-27 17:23:09: findKernel crashed [t [Error]: Invalid response: 503 Service Unavailable
      at /home_local/user/.vscode-server/extensions/ms-toolsai.jupyter-2021.8.1054968649/out/client/node_modules/@jupyterlab/services.js:3:492374
      at runMicrotasks (&lt;anonymous&gt;)
      at processTicksAndRejections (internal/process/task_queues.js:93:5)
      at async m.getRunningKernels (/home_local/user/.vscode-server/extensions/ms-toolsai.jupyter-2021.8.1054968649/out/client/extension.js:90:227410)
      at async Promise.all (index 0)
      at async y.listKernels (/home_local/user/.vscode-server/extensions/ms-toolsai.jupyter-2021.8.1054968649/out/client/extension.js:90:468689)] {
    response: N [Response] {
      size: 0,
      timeout: 0,
      [Symbol(Body internals)]: { body: [PassThrough], disturbed: false, error: null },
      [Symbol(Response internals)]: {
        url: 'http://distant_adress:8888/api/kernels?1627399389810',
        status: 503,
        statusText: 'Service Unavailable',
        headers: [I [Headers]],
        counter: 0
      }
    }
  }
  </code></pre>
  <p>Where distant_adress is the address that is given by the command <code>jupyter notebook list</code> and <code>user</code> is simply my username on linux.</p>
  <p>I already tried :</p>
  <ul>
  <li>Restarting jupyter and changing the address : does not work</li>
  <li>Since I also have a port forward, go to the local address on my computer and not on the server : does not work (however jupyter on my machine works well with the port forward)</li>
  </ul>