Jupyter nbextensions does not work as of jan 2021 update notebook

[cems2]

Problem: Nbextensions configurator was working just fine up until an anaconda update in jan 5 2021. Now it is broken.
1. There is no nbextension configurator tab visble on jupyter notebook server page
2. http://8888:/nbextensions gets a blank page (but not a missing page error)
3. installing a new NBextension works but enabling gets a validation error that there is no main.js (or whatever was stated as the entry point)

That is to say both the NBextension is not working nor are any extensions.

Reproucibility: 100% reproducible.
I have tried this on 3 different computers with Mint distro Linux as well as on clean new installs of Anaconda in newly created users. always the same result.
Additionally on two machines I have, this was working just fine up until I updated anaconda.
so this is perfectly reproducible on many machines and definitely related to the anaconda update.

Workaround: I workaround it by downgrading the notebook version
pip install notebook==6.1.5
then the enable from the command line may not work but the enable from the nbextensions tab does work

Regression:
This could be related to a missing mathjax library discussed here:
https://discourse.jupyter.org/t/the-static-notebook-js-mathjaxutils-js-is-missing/7303

related issues
And this mode of error is similar to an ancient issue #117 however that was for the notebook 6.0.3 (may 2020) which is older than even 6.1.5. So it appears an older error that was eventually fixed in 6.1.5 has now re-appeared in the newest jan 5 2021 version of notebook.

[tfiers]

Can confirm this on Windows as well (exact same problem & workaround as described by @cems2).
Previously installed extensions seemed to keep working though.

[AaronStiff]

I believe @takato1314 has described a simpler solution in #125 which does not involve downgrading. The issue seems to have been caused by a moved file.