I think you should add "traceback info may contain clues" in your howto so that googleing for this very specific message would be successfull. Ok, I'll search my message in the newsgroup and include it.

Also your solution says "I will assume that you have also a fresh installation of Zope 2.6.0 without anything on it and of course your old Zope 2.5.1".

As specified in my previous message, my problem is (was I hope) that I don't have my old Zope 2.5.1 ! This means that your solution, in case I had found it, was unusable at that time, i.e. before I tried to downgrade. Sorry for that, I thought you had a backup.

PS : I already know who I have to blame for not having a backup, thank you very much. Who me? I think it's a responsibility of each administrator to do a backup before doing a software upgrade.

Anyway, now I just remmember that somebody disscussed just some days ago a way of correcting mistakes if you have already upgrade your zope. He said that you had to export as XML, look for a string (If I'm not bad it was something like <object>, you've to look the archives) erased it and reimport the object again. Regards, Josef.