SyntaxError: Unexpected token < in JSON

This site works perfectly well on local machine (Open Server) and another live server running Apache. But Price Calculator fails to load on server running nginx (attached link). But I’m not sure if the problem is in the nginx.

I get this error in console:
AWS Price Calculator Error: SyntaxError: Unexpected token < in JSON at position 0

And if I’m not wrong the AJAX repsonse looks like:

<br />
<b>Warning</b>: Invalid argument supplied for foreach() in <b>/var/www/miralls/data/www/miralls.ru/wp-content/plugins/woo-price-calculator/admin/awspricecalculator/Helper/CalculatorHelper.php</b> on line <b>180</b><br />
{“errorsCount”:0,”errors”:[],”price”:”<span class=\”woocommerce-Price-amount amount\”>27720\u00c2\u00a0<span class=\”woocommerce-Price-currencySymbol\”>\u00d1\u0080\u00d1\u0083\u00d0\u00b1.<\/span><\/span>”,”outputFields”:”<br\/>”,”conditionalLogic”:{“5″:1,”6″:1,”4″:1,”3″:1,”1″:1,”2″:1,”price”:1}}

And of couse the response is not a valid JSON object.

Any ideas how to fix this issue? (Except of using Apache server)

The page I need help with: [log in to see the link]