Help Customer Can't Add Products To Cart (4.3.3 And 4.3.4)

We have a test site (devx) and production site (www). The devx site has been upgraded to 4.3.4 while www is still at 4.3.3. Both versions will not allow shopper to add products to cart. This is now an emergency so any ideas on why this is happening and how to fix appreciated. Thanks for any help!

What browser, what errors if any do you get, and whats the url

and does this apply to you

Thanks for reply. I use Mac Chrome, Firefox. It happens on PC systems as well. No error message, just a spinning gear progress indicator that disappears without putting anything in the cart. URL is:

That product is a dummy but it happens for all products. First noticed some time after after upgrading to 4.3.3.

Regarding the link, I really don't know if it applies. Maybe, but I don't see a smartoptimizer cache folder.. I don't use IE at all but others who do are having the same problem. We had installed the mods mentioned in the CS-Cart blog on optimizing server: Redis, PHP 5.6, APC, Opcache and the problem manifested with this setup. We undid all the PHP changes and it's still present.

Try to disable 3rd party modules one by one.

If I disable ajax, everything works correctly: