Just in case - if anybody has same problem.. this is my solution - contact your hosting provider FIRST!!!!
In my case it was problem with hosting.. SEO URL was also not working with "add to cart" and my hosting company was telling me that it was internal zen cart problem with ports and I tried to fix it for days.. Finally, when I tried everything and could not fix it and hosting refused to help me - I changed my hosting company to GoDaddy - and big surprise - everything was working perfect!!
Bookmarks