here what I did :
Finaly downgrade to WP e-Commerce Version 184.108.40.206
No more error from standard class, steel not completely working but now customer and admin receive confirmation mails.
The problem look located in the way the plugin try to connect to google drive to write log and is not in the zend library because it can write sales.It make the module exit so no mail and a blank page.
My customer can't wait and we together lost to much time and money so here is what I did if I can help others one lost in that bad situation :
Create 2 pages one for accepted transaction, one for cancel transaction
In the WPCB option
uncheck Add Sales to Google spreadsheet
In the Atos options living automatique response to blank or put a wrong path to automatic_response_url so automatique response is not called this way.
put automatic_response.php in the normal return
put the link to my cancel page in the cancel return.
In the automatic_response.php near ligne 148 after :
put code to call my accept page (called merci):
$url = site_url()."/merci"; (here in place of merci your page)
The site will work this way for the time I write my own plug in or 6www make the correction.
Finally free solution can be very expensive even if you are ok to pay. Should be great if 6WWW tell us witch configuration is working and alert new users for incompatibility.