Starting slash in form action interferes with base-href usage
In file pi1/couponfile.tmpl there's a section COUPON_FORM where the form action has a starting slash. In case you put a TYPO3 installation in a subdirectory and although you set the right config.baseURL value you will encounter the problem that by submitting the form you are send to the root directory of the hosting (which then most probably results in a 404 error).
Long story short:
Simply remove the starting slash from the form action attribute!