Redirect woocommerce checkout page

Hello,
I set up a gateway plugin provided by a company to allow me collect money in different ways in my country.
After adding the plugin and connect it to their website and make initial test no problems happend.
After two days; when choosing payment method got the following error

“SyntaxError: Unexpected token < in JSON at position 0” when stopped ithemes security pro the error gone but other error still exists which is customer redirected to homepage and erase product from cart when going to pay.
I don’t know what is the problem?!
This problem also affect the default woocommerce gateways.

Hope to help me.. Thanks