The Python stack trace is awful for the user, but it's there as a development aid. If a user sees it, something is badly wrong in Obnam.
Are you sure you have the right version of Obnam? How do you check that? Obnam is supposed to report the right version number in the log file or with "obnam --version", and if it isn't 1.0 then something's wrong.
Do you have the right version of the Larch Python library installed? That stack trace looks like there's an old version installed. Are you running on Debian? Ubuntu? Somewhere else? The Ubuntu PPA is in the process of being updated to fix a few problems with wrong versions. If you're on either Ubuntu or Debian, what is the output of "dpkg -l obnam python-larch"?