Checkout: W3 Total Cache conflict
The problem is caused by both our Shopping Cart plugin and the W3 Total Cache plugin enabling GZIP compression on your WordPress site. This causes a compression conflict.
The solution is to turn off the Gzip compression inside either of these two plugins.
Disable in the Shopping Cart plugin
To disable Gzip compression in the Shopping Cart plugin, go to Checkout > Configuration > General Configuration. The very last setting named “Enable Gzip Compression” can be set to “No” and then you must save the configuration.
Disable in the W3 Total Cache plugin
To disable Gzip compression in the W3 Total Cache plugin, go to Performance > General Settings in the WordPress dashboard. Scroll down to “Browser Cache” where you can untick/uncheck the checkbox and save the settings.