I have looked into this a bit and don't see a good reason for it displaying on one part of the site but not this other part of the site. To help debug the issue you will need to open the galleries > grid_lightbox > index.php file and add this code in just after line 42:
print("the first fullsize URL is: " . $gallery['images']['fullsize']['url'] . "<br/><br/>the php_self is " . $_SERVER['PHP_SELF']);
This will print out the full size image URL as well as the php_self setting on both places where the gallery code has been placed. We should then be able to compare the values on the one that is working versus the one that is not working to see what the difference is. Please give this a try and let us know when you have added the code so that we can take a look.