Cannot change to a different GUI toolkit: notebook - Warning in Jupyter

As ImportanceOfBeingErnest pointed out in the comments, my problem was related to the backend already being set when jupyter starts. However, as mentioned in this post, the backend needs to be set in the ipython_config.py, not the jupyter_notebook_config.py.

After restarting the notebook server, I can switch between backends as described here.

You must put %matplotlib widget in the very beginning of the Jupyterlab