500: Server internal error
In rare occasions you may this error when spawning a new JupyterLab instance or reconnecting to an existing one. In this case, you will need to manually cancel the failed instance and start a new one.
You may go to the JupyterHub home at https://jupyterhub.ecmwf.int/hub/home and click on the red button "Stop My Server". After a few seconds, you will be able to start a new one with the desired settings.