nbclassic 0.2.6 pyhd3eb1b0_0
@tburnett you have an outdated nbclassic version which does not work with current jupyter_server release. You need to update it to version 0.2.8 or newer, see https://github.com/jupyterlab/jupyterlab/issues/10228#issuecomment-868459759 (note: you want to specify -c conda-forge channel, depending on from which channel you installed jupyterlab).