The validation_png.php file has just been modified in line 30 :
define(ANTI_ROBOT_IMAGE_PHP_BITMAP_FONT, 5);
replaced by
define('ANTI_ROBOT_IMAGE_PHP_BITMAP_FONT', '5');

With this mod the validation code appears in the box. Well, well, 4 hours to find this ;-)
Hope it could be usefull for someone.

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