Same thing was happening to me, but I think I figured out what the problem was. First of all, I'm using Weebly to host my store (which Square just bought), so not sure if it's a Weebly specific problem. But I had recently purchased a new URL that I was using to forward to my store. I had the forward set to "mask", meaning it showed the new URL name in the browser instead of the "real" URL of the store. When I disabled masking for the URL forwarding (in godaddy), it immediately solved the problem. I figured it out when I noticed that the masked URL showed a Not Secure notification in one of my browsers. So, I'm assuming that's the problem with using a masked forward. Perhaps there's a way to provide a secure forward using an SSL certificate, but for now, I'm just having the forward send people to my old store address.
e.g.: My store is www.ocalasgoodlife.com/cepnetwork and I purchased a new URL, www.cepnetworkmag.com and set it to masked forward. So when people went to cepnetworkmag.com the browser displayed ** cepnetworkmag.com ** while they were on ocalasgoodlife.com/cepnetwork. But that since it was lacking the https: (meaning, it was SECURE), it messed up the ability to add items to the sopping cart. As soon as I removed the masked forward, the shoping cart worked again.
