I've found the mistake :
Two things in fact :
In the shop.cart.tpl.php :
I remplace the line :
echo vmCommonHTML::getGreyBoxPopupLink( $href2, $text, '', 'Idées couture - Commande en ligne ', $class_att, 600, 640, $href );
by :
echo vmCommonHTML::getGreyBoxPopupLink( $href2, $text, '', 'Idées couture - Commande en ligne ', $class_att, 600, 640, $href2 );
And in the file htmlTools.class.php :
I remplace the line 1808 :
$link = vmCommonHTML::hyperLink( $no_js_url, $text, $target, $title, $attributes.' onclick="try{ if( !parent.GB ) return GB_showCenter(''.$title.'', ''.$url.'', '.$height.', '.$width.');} catch(e) { }"' );
by :
$link = vmCommonHTML::hyperLink( $no_js_url, $text, $target, $title, $attributes.' onclick="try{ if( !parent.GB ) return GB_showCenter(''.$title.'', '/'.$url.'', '.$height.', '.$width.');} catch(e) { }"' );
So it's ok now for me !!!