Urgent Fix for Simple Gmail Notes (2020-12-09)
Update 2020-12-10 6:00 UTC - The new extension is now available on Chrome store.
There is some change with Gmail UI today and caused problem, we just released a new version (22.214.171.124)
It's now approved by Google side, you may want to manually update the extension:
Alternatively, you may use the Firefox extension for now, you could always retrieve the previous notes as far as you logged into the same Google account: