Thinking you might have pinned the wrong version of nbconvert or something in your conda recipe.
conda create -n arcgis arcgis -c esri -y
conda activate arcgis
conda install geopandas -c conda-forge -y
pip install notebook
Opening any notebook throws 500 : Internal Server Error untill nbconvert is updated
pip install --upgrade nbconvert
Pro installs arcgis and it's pinned version of nbconvert is 5.6.* and notebook 5.7.*
Perhaps some issues mixing conda-forge and pip install is causing issues.