Zen Cart E-Commerce Shopping Cart

The statements you posted are not consistent with the experience of the
majority of users. It would seem that there must therefore be a problem
with your server/hosting environment which is causing either an
incompatibility ... perhaps there is a mod_security rule causing a
problem?
You could ask your hosting company for access to the server's errorlogs to
determine the cause of the "Forbidden Access" message.
You might also try using one of the newer versions of Zen Cart, since
v1.3.6 is very old and no longer officially supported.