Error generating image, no error log
Problem is - you are trying to generate styled image, but you do not provide security token. In Drupal 7.20 and later security token is required so images are only generated when Drupal site itself tries to use them. To turn it off, just add the following line to your settings.php file:

$conf['image_allow_insecure_derivatives'] = TRUE;


Solution taken from this answer and 7.20 release notes.