Contact form 7 invalid json

Hello Support Team,

I have used the contact form 7 plugin on my website and created one registration form and when I submit that form it displays an error.

This error

{code: "invalid_json", message: "The response is not a valid JSON response."}code: "invalid_json"message: "The response is not a valid JSON response."__proto__: Object
(anonymous) @ index.js?ver=5.4:1
Promise.catch (async)
f @ index.js?ver=5.4:1
(anonymous) @ index.js?ver=5.4:1

Error screenshot – https://i.imgur.com/wmP7wk1.png

Please check it and let me know asap.

I am waiting for your reply

Thanks
Nilesh

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