title: Jupyter Lab "404 : Not Found You are requesting a page that does not exist!"
body: <p>Jupyter Notebook and Jupyter Lab worked well until I updated packages and set environments.
  After that, JupyterLab 3.0 doesn't open. It shows 404 and an error message:</p>
  <pre><code>[I 18:21:13.532 LabApp] JupyterLab extension loaded from 
  C:\Users\Alfonso\AppData\Roaming\Python\Python38\site-packages\jupyterlab
  [I 18:21:13.532 LabApp] JupyterLab application directory is C:\Users\Alfonso\AppData\Roaming\Python\share\jupyter\lab
  [W 18:21:13.533 LabApp] Error loading server extension jupyterlab
      Traceback (most recent call last):
      File &quot;C:\ProgramData\Anaconda3\lib\site-packages\notebook\notebookapp.py&quot;, line 2036, in init_server_extensions
          func(self)
      File &quot;C:\Users\Alfonso\AppData\Roaming\Python\Python38\site-packages\jupyterlab\extension.py&quot;, line 226, in load_jupyter_server_extension
          add_handlers(web_app, config)
      File &quot;C:\ProgramData\Anaconda3\lib\site-packages\jupyterlab_server\handlers.py&quot;, line 162, in add_handlers
          handlers.append((url_pattern, LabHandler))
      AttributeError: 'NotebookWebApplication' object has no attribute 'append'
  [I 18:23:27.282 LabApp] Serving notebooks from local directory: C:\Users\Alfonso
  [I 18:23:27.282 LabApp] Jupyter Notebook 6.3.0 is running at:
  [I 18:23:27.282 LabApp] localhost:8889/…
  [I 18:23:27.283 LabApp] or 127.0.0.1:8889/…
  [I 18:23:27.283 LabApp] Use Control-C to stop this server and shut down all kernels (twice to skip confirmation).
  [C 18:23:27.370 LabApp] To access the notebook, open this file in a browser: file:///C:/Users/Alfonso/AppData/Roaming/jupyter/runtime/nbserver-5876-open.html
  </code></pre>
  <p><a href="https://i.stack.imgur.com/WkwQj.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/WkwQj.png" alt="enter image description here" /></a></p>
  <p>I changed the browser (Firefox instead of Chrome), followed the suggestions in another post (<a href="https://stackoverflow.com/questions/48948259/how-to-solve-404-error-of-jupyter-lab">How to solve 404 error of jupyter lab</a>), but neither worked. Also, I've tried to fix the problem by installing as administrator and as user.</p>
  <p>I was wondering if there's a solution that not involves reinstalling all Anaconda distribution.</p>
  <p>Installed versions:</p>
  <ul>
  <li><code>jupyterlab</code> 3.0.11</li>
  <li><code>jupyterlab_server</code> 2.4.0</li>
  <li><code>notebook</code> 6.3.0</li>
  </ul>