JSON sent after page is submitted is mangled

The JSON returned on submission is :

add_spam_logArray1kkk{"into":"#wpcf7-f48-p49-o1","status":"mail_sent","message":"Thank you for your message. It has been sent."}

This causes the form to appear not to submit, because it’s not valid JSON.

I’ve tried changing several settings and can’t find what would be breaking it so badly.

Any one know ?

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