Making an XHR request from AMP-HTML contact form to Laravel 5.6 Unexpected token < in JSON at position 0

I'm getting this error when making an ajax request from an AMP form to the Laravel 5.6 controller. I used a similar approach like in this question: Making an XHR request from AMP-HTML contact form to Laravel 5.4. 500 ERROR

But I get the following error in my console when I hit submit on a normal search form: [amp-form] Failed to parse response JSON: SyntaxError: Unexpected token < in JSON at position 0

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