I just verified that the api_key in the mailman-hyperkitty.cfg matches the 'MAILMAN_ARCHIVER_KEY' credential in mailman-web.py - I had to set that manually in the .cfg and thought maybe I messed that up. I also took a look at the MAILMAN_REST_API_... credentials in mailman-web.py (that's the settings_local.py file on the Ubuntu setup) and verified they matched the webservice credentials in the mailman.cfg, thinking that maybe this is a permissions issue related to the API?
The error message says HyperKitty failure on https://udfcd-lists.org/mailman3/hyperkitty/api/mailman/urls. I'm not sure if that's a clue or not - I can see the Authorization Required message at that address, to be sure. I looked into the referenced .py file at the line numbers referenced, and I can see the lines of code referenced in the error message, but I don't know why they are failing. I suspect I don't have a key or password or some other credential set properly in some file...but where?