Hi
Please try adding the following to the wp-config.php file of your site
/*disable Imagick/
define('NGG_DISABLE_IMAGICK', TRUE);
We are trying to narrow down the issue to the imagick module version. After adding this, make sure to refresh any and all site cache. Then try adding a new gallery and see how the images come up.
Also you can create new thumbnails for the broken images from Manage Galleries, Open a gallery and then select the option to create new thumbs from the bulk actions or for each individual image.
Let me know how it went.
Thanks!