Was a bit of a bumpy ride: apparently when upgrading from Zope 2.7.b3 to any later version, compatibility with the Data.fs file is not given! But after reverting back to the previous version, exporting the plone sites via ZMI, re-upgrading (thank God for symlinks!) everything worked like a charm and I can now, once again enjoy the good feeling only a true geek can comprehend of being totally and utterly up-to-date. For a while... ;-)

Needless to say, I've updated the corresponding section of the FreeBSD Cookbook, so for all those other update-hungry Zopeistas out there: come along...