Jupyter notebook Nbextensions shows only spinning reload circle

I installed WSL Ubuntu 22.04.3 LTS.

I had not any problem with Nbextensions before I reinstall jupyter notebook.

I totally reinstalled Ubuntu (WSL Ubuntu) and jupyter notebook (See below code).

pip install notebook==6.1.5

Then, I entered below code as before.

pip install jupyter_contrib_nbextensions

jupyter contrib nbextension install --user

jupyter nbextension enable varInspector/main

As can see above, All looks fine for installing Nbextensions.

However, when I go Nbextensions tab, I see only spinning reload circle as below.

I searched about this problem and many people are talking about JSON.

However, I actually don't know much about it. So I entered what it is written.

In ".jupyter" folder, there is "nbconfig" folder and "jupyter_nbconvert_config" file.

In that folder, there are 'notebook' and 'tree' files.

The notebook file has code as below.

{
  "load_extensions": {
    "nbextensions_configurator/config_menu/main": true,
    "contrib_nbextensions_help_item/main": true,
    "varInspector/main": true
  }
}

The tree file has code as below.

{
  "load_extensions": {
    "nbextensions_configurator/tree_tab/main": true
  }
}

'jupyter_nbconvert_config' file has code as below.

{
  "version": 1,
  "Exporter": {
    "extra_template_paths": [
      ".",
      "/home/park/anaconda3/envs/surging/lib/python3.9/site-packages/jupyter_contrib_nbextensions/templates"
    ],
    "preprocessors": [
      "jupyter_contrib_nbextensions.nbconvert_support.CodeFoldingPreprocessor",
      "jupyter_contrib_nbextensions.nbconvert_support.PyMarkdownPreprocessor"
    ]
  }
}

What should I have to do with it?

I found some errors were occured in Ubuntu terminal during opening jupyter notebook.

So I tried to find solution and below code makes all normal.

pip install "jupyter-server<2"

Now I can use nbextensions.