I received the same error and doing the following worked for me:
Check for the blinker package
sudo apt list python*blinker*
remove it through apt
sudo apt remove python3-blinker
run the command that Mail-in-a-Box setup uses to update the packages via pip (it’s a long one all on one line)
sudo /usr/local/lib/mailinabox/env/bin/pip install --upgrade rtyaml "email_validator>=1.0.0" "exclusiveprocess" flask dnspython python-dateutil expiringdict gunicorn qrcode[pil] pyotp "idna>=2.0.0" "cryptography==37.0.2" psutil postfix-mta-sts-resolver b2sdk boto3
re-run Mail-in-a-Box setup.
Just out of curiosity, I had Python and Pip errors that I had to manually resolve when moving to v60/v61, and I’m pretty sure this is related to that for me. Wish I documented what it was exactly. Did you experience anything strange when moving as well?