I also tried exporting one of the broken ZClass products from a 2.5 Zope

Is your ZClass a subclass of ObjectManager? If this is the case, then you have to delete the references in "Subobjects", export your ZClass, import it on 2.6.x, and add the references manually. More info: http://www.zope.org/Members/jmeile/upgradingZope Regards, Josef