If it is working on one site on the server and not the other, then it is unlikely to be either the server or IH3 that is causing the problem

on the non-working site
1) What lightbox are you using (full name and version)?, try to get imagehandler 3 working before adding any light box mod
2) what are the permissions on your images folder
3) What are the permissions on your bmz_cache folder
4) are any files/folders being created within the bmz_cache folder?


can you confirm that these files are on your server

includes/modules/ArtSite/main_product_image.php

includes/functions/extra_functions/functions_bmz_image_handler.php

What happens if you delete the .htaccess file in the bmz_cache?