Thanks for trying to help but the problem is that I did not try to create any period... all I did was to try to change value in the accounting page of the setting menu. Whatever field I try to update is bringing this error.

I think is a database error, try a clear new installation. If you speak french, this is my method : http://dipleo.fr/installer-openerp-rapidement/

Thanks for the info ! However I was able to isolate the problem to a narrower field of trouble shooting. My problem arise only when, after a fresh install with new database, I answer to the accounting wizard and ask for a period starting February first. If I leave it at the default January first all seems to be working as expected. I did not tried other period dates but I suspect the wizard to be messing stuff.