I got a validation error when closing my POS session in openERP 7.

After having tried different settings and re-installed fifty times my DB, I finally initialized a last DB and, instead of choosing the specific french chart of accounts as usual, I selected the custom chart of accounts option then, I set the parameters of my POS payment's method , open my POS session, made a sale, and, what a surprise, when I closed my POS session, it worked !

I guess there must be a bug or a malfunction with this specific chart of accounts. Does somebody know about this, or may kindly debug the code for me within this specific context (point of sale with french accounting set) ?