Thank you @angus Good to know about /site.json where I do see all categories. I found the problem! It was totally PEBKAC.
In the plugin settings, for Discourse URL, I had entered an http:// url not https:// - note the example below the field perhaps should reflect this.
Changing our URL to https://connect.orglobal.org made everything work.
I am embarrassed for missing something so obvious. Case closed.