Uncaught SyntaxError: Unexpected token < in JSON at position 0

When submitting a Data Access Request with latest version 1.4.3 the AJAX call gives an error:

Uncaught SyntaxError: Unexpected token < in JSON at position 0
at JSON.parse (<anonymous>)
at XMLHttpRequest.<anonymous> (front.js?ver=1541869345:250)

More details:

REQUEST
Request URL: https://xxxxxxxxx/wp-admin/admin-ajax.php
Request Method: POST
Status Code: 400
Remote Address: xxx.xxx.xxx.xxx
Referrer Policy: no-referrer-when-downgrade

RESPONSE
accept-ranges: bytes
content-length: 1393
content-type: text/html
date: Sat, 24 Nov 2018 15:48:42 GMT
etag: “xxxxxxxx”
host-header: xxxxxxxx
last-modified: Wed, 25 Jan 2017 13:34:32 GMT
server: nginx
status: 400

FORM DATA
action: wpgdprc_process_action
security: xxxx
data: {“type”:”access_request”,”email”:”[email protected]”,”consent”:true}