Unable to install the tracker on Zope 2.6

Hi all,
I just tried to install the Zope tracker on Zope 2.6 (i previously had it on
Zope 2.3) during an upgrade. When I import the Tracker.zexp file into the
products folder, I get an error; this is the traceback:

Traceback (innermost last):
  Module ZPublisher.Publish, line 98, in publish
  Module ZPublisher.mapply, line 88, in mapply
  Module ZPublisher.Publish, line 39, in call_object
  Module OFS.ObjectManager, line 530, in manage_importObject
  Module OFS.ObjectManager, line 546, in _importObjectFromFile
  Module ZODB.ExportImport, line 79, in importFile
  Module ZODB.Transaction, line 222, in commit
  Module ZODB.Transaction, line 195, in commit
  Module ZODB.Transaction, line 256, in _commit_objects
  Module ZODB.Connection, line 276, in commit
  Module ZODB.ExportImport, line 148, in _importDuringCommit
SystemError: Failed to import class rfind from module App.FactoryDispatcher

The tracker was installed on a fresh Zope 2.6 copy, installed from the
binary distribution on a RH 8 Linux.

It seems that something (very stupid) has broken.
Could anyone help me ? I am neither a Zope nor a python expert (even if I
already installed and used some products), but I appreciate the tracker and
would like to use it with new Zope versions.

Thank you

Paolo Marini

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