thank you, very much, David; I opened a thread on the ZODB list and, according to Jim, seems we have some missing objects from the database.
he suggested to add pack-gc false to the <filestorage> section of the ZEO server configuration to disable garbage collection.
I did so, and I was able to pack the ZODB to 7GB from 9GB.
I was wondering if it's possible to add that to the buildout configuration so I don't lose it on the next update.
Jim also suggested me the following packages to try the garbage collection and see if we can get more information on the missing object:
but they lack decent end-user documentation; does anybody here has used them?
UPDATE: @alert just told me to use pack-gc = false on my ZEO server part according to plone.recipe.zeoserver.