Meanwhile going ahead slowly. This resolved issue helped a lot:
https://gitlab.com/mailman/hyperkitty/-/issues/248
'charset': 'utf8mb4' into /etc/mailman3/mailman-web.py helped and now a message with utf8mb4 emoji char is archive properly.
But reading that particular msg in Hyperkitty spits out a new error:
Internal Server Error: /mailman3/hyperkitty/list/test1@dwatt.com/thread/OIHKKHNGNL6HF2SMFJZXSS6TABO2VK6Z/
AttributeError at /hyperkitty/list/test1@xxxxx.yyy/thread/OIHKKHNGNL6HF2SMFJZXSS6TABO2VK6Z/ 'NoneType' object has no attribute 'subject'
Request Method: GET Request URL: https://xxxxx.yyy/mailman3/hyperkitty/list/test1@xxxxx.yyy/thread/OIHKKHNGNL... Django Version: 2.2.25 Python Executable: /usr/bin/uwsgi-core Python Version: 3.9.2 Python Path: ['.', '', '/usr/lib/python39.zip', '/usr/lib/python3.9', '/usr/lib/python3.9/lib-dynload', '/usr/local/lib/python3.9/dist-packages', '/usr/lib/python3/dist-packages', '/usr/lib/python3.9/dist-packages'] Server time: Wed, 16 Feb 2022 21:27:14 +0100
Googling the mailman-users list I found another case where hyperkitty_thread table had a NULL starting_email_id and yesss! It has a NULL in starting_email_id really!
Now the billion$ question: what this particular msg with an utf8mb4 char has to do with starting_email_id??
Sanyi