POSSIBLE FIX:
Had this same issue. My issue stemmed from the fact that I have multiple domains pointing to the same website (ie. mywebsite.com, myawesomewebsite.com). Sometimes there are redirection issues where when you press the Add to Cart or Proceed to Checkout it tries to jump between the two domains. This causes a security issue with Javascript (annoying but for good reasons).
I fixed this by adding a permanent redirect with my host and also by going into the wordpress settings and making sure the domain urls are set to my primary domain (CAUTION: IF YOU PUT IN THE WRONG DOMAIN IT WILL LOCK YOU OUT OF YOUR WORDPRESS AND THEN YOU WILL HAVE TO FIX IT THROUGH THE CONFIG FILE). I also had to go into my database and change the home url to my primary domain.
Hope this helps!
