As you will notice, in version 0.5.1 of jupyter_contrib_nbextensions most of the nbextensions are compatible with versions 4.X and 5.X. For example look at this extension:
It you uncheck the option “disable configuration for nbextensions without explicit compatibility“, you will most probably have no problem using any extension. I guess that the developers haven’t yet tested the jupyter_contrib_extensions against the new notebook version 6.X which was recently launched. If you want to have a stable and tested environment, you can roll back to a previous jupyter notebook version. The most recent compatible one is 5.7.8. Just uninstall current notebook and install that specific version by executing:
pip uninstall notebook
pip install notebook==5.7.8