I just switched Hosting Companies (to one Zen Cart's Recommended ones which I used for some other sites and loved! Hence me switching...) The site has been down for about a week (BAD!) and finally got the dedicated IP and SSL ordered and I assume installed.
My Log In and Subscribe pages are working and going to https://, but the Add To Cart button just gives me a blank white page.
My configure files are set to use SSL for the catalog.



Why don't you just start by dealing with the specific filenames it's mentioning? There are FAQs dealing with "cannot modify header information" and "headers already sent", such as: 
