You could try downloading and installing a working duplicity, before the MIAB install. I’d try:
wget https://launchpad.net/~duplicity-team/+archive/ubuntu/duplicity-release-git/+build/30958892/+files/duplicity_3.0.5.1-ppa202506251611~ubuntu22.04.1_amd64.deb
apt install ./duplicity_3.0.5.1-ppa202506251611~ubuntu22.04.1_amd64.deb
apt-get hold duplicity
keep an eye on https://discourse.mailinabox.email/t/duplicity-oops-they-did-it-again/16051 … something will appear when a working duplicity update is available.