Hi,
I noted that after installing IH 4 on the top of a standard Zen Cart 1.5.1, the webshop administrator can't access anymore to update an existing product or add a new one. There is no problem to browse all the categories and subcategories though. So, the denial comes only when trying to access the product form itself to update the product info. Superuser has no problems to edit and add new products.

There hasn't been done anything else than installed the IH4 lately for the shop and before that administrator level could update products without a problem.