How did you upgrade? The error says you do not have a Mailman 2.1.14 Defaults.py.
Did you by chance make changes in your prior Defaults.py and then restore the old one after the upgrade? If so, this is exactly why you should never change Defaults.py and make all customizations in mm_cfg.py.
A better work around would be to simply add
ALLOW_SENDER_OVERRIDES = Yes