I'm having this issue and I already know the crupt, but still couldn't find a solution.
Open the file includes/functions/functions_general.php
Inside the function zen_redirect - line 37 paste the following:
Then go to your the product's page
Clear your browser cache, reload the product's page (don't go to other pages) and try adding the product to your cart.
See if you get a redirect from line 33 of the file init_sanitize.php