Check whether HTTP requests are proxied. This is the case if the kernel is started from Jupyterhub and the Python package "jupyter_server_proxy" is installed.
Source:R/httpd.R
http_is_proxied.RdIf requests are proxied and the HTTP server of the R session listens at port `<PORT>` then the appropriate URL is of the form `<JUPYTERHUB_SERVICE_PREFIX>/proxy/<PORT>` in addition to (or instead of) `http://localhost:<PORT>`.