Menu bar issue in jupyter notebook

Seemingly without reason, all the button icons in some of my Jupyter notebooks have disappeared and this is what shows up instead:
jupyterissue

Looking at the page source in the browser I see the following seemingly related warning:

menubar.js:263 actions jupyter-notebook:find-and-replace does not exist, still binding it in case it will be defined later…

The issue does not appear in all notebooks, but once it appears it says: refreshing the browser, restarting the jupyter server etc do not fix it.

I am using pyViz panel and some custom css for formatting tables, not sure if related but the issue seems to have started when I started doing this. Example.

import panel as pn
cssf = ["./resources/pure-release-1.0.1/pure-min.css", “./resources/fontawesome-free-5.12.0-web/css/all.min.css”]
pn.extension(css_files=cssf)

Does anybody else have seen this before ?