I followed the instructions on this post , http://drupal.org/node/158043 and Enabled Error Reporting .
so I now got the following error ...
Fatal Error : Call to undefined function entity_language() in ....\includes\commerce_product.forms.inc on line 43

this is also mentioned in http://drupal.org/node/1821718 , and as I read I have to upgrade to Drupal 7.15 or later (I am using 7.14 at the moment ...)