Humm after some more investigations, it seems that my mailman3 installation via debian packages is completely mixed up :-( This would explain the issue if the REST API is using 3.2.2 code and postorius is expecting 3.3.5...
drwxr-xr-x 29 root root 4096 Nov 18 2021 /usr/lib/python3/dist-packages/mailman drwxr-xr-x 2 root root 4096 Nov 18 2021 /usr/lib/python3/dist-packages/mailman-3.2.2.egg-info drwxr-xr-x 8 root root 4096 Nov 18 2021 /usr/lib/python3/dist-packages/mailmanclient drwxr-xr-x 2 root root 4096 Nov 18 2021 /usr/lib/python3/dist-packages/mailmanclient-3.2.2.egg-info drwxr-xr-x 4 root root 4096 Apr 13 14:29 /usr/lib/python3/dist-packages/mailman_hyperkitty drwxr-xr-x 2 root root 4096 Apr 13 14:28 /usr/lib/python3/dist-packages/mailman_hyperkitty-1.1.0.egg-info drwxr-sr-x 30 root staff 4096 Apr 13 09:32 /usr/local/lib/python3.8/dist-packages/mailman drwxr-sr-x 2 root staff 4096 Nov 18 2021 /usr/local/lib/python3.8/dist-packages/mailman-3.3.5.dist-info drwxr-sr-x 9 root staff 4096 Nov 18 2021 /usr/local/lib/python3.8/dist-packages/mailmanclient drwxr-sr-x 2 root staff 4096 Nov 18 2021 /usr/local/lib/python3.8/dist-packages/mailmanclient-3.3.3.dist-info drwxr-sr-x 4 root staff 4096 Apr 12 20:47 /usr/local/lib/python3.8/dist-packages/mailman_web drwxr-sr-x 2 root staff 4096 Nov 18 2021 /usr/local/lib/python3.8/dist-packages/mailman_web-0.0.5.dist-info