I'm having the same issue, running 3.2.0. Just started today afaik...
It may be related to this python module in the default python package, but I haven't been able to resolve it yet: https://github.com/jupyter/notebook/pull/7051
It turns out that in my case, I had a python installation outside of Arc Pro that was more recent, and that seems to have been the source of my problem. Once I removed the other install, Notebooks would open in Arc Pro as before.
Just checking if there was an update on how to fix this?
I got same error on 3.2.2 but when I downgrade my Pro to v2.9.6 my notebooks are opening fine.
In addition, On v 3.2.2 I cannot clone the default env as well