Ajax Error: Unexpected token < in JSON at position 0 on WordPress Contact Form 7 submit

I have a weird issue when posting an contact form. The loading icon keeps loading and the form does not submit. The emails are sended and my before_send_mail functions also works. The weird thing is when i uncomment the before_send_mail function, it shows no errors. So it's probably something from my code. However the frontpage is not changing status and keeps showing the loading icon. The error message sais: ``` <div class="ajax-error">Unexpected token &lt; in JSON at position 0</div> ``` Can you guys help me out? Below you'll find the before_send function. ``` add_action( 'wpcf7_before_send_mail', 'form_to_crm' ); function form_to_crm( $cf7 ) { $wpcf7 = WPCF7_ContactForm::get_current(); /* Uw naam => first_name */ $first_name = $_POST["your-name"]; /* Bedrijf => company_name */ $company = $_POST["bedrijf"]; /* Email => email */ $email = $_POST["email"]; /* Adres => address */ $address = $_POST["adres"]; /* Nummer* => number */ $number = $_POST["huisnummer"]; /* Postcode => postcode */ $postcode = $_POST["postcode"]; /* Woonplts* => city */ $city = $_POST["woonplaats"]; /* Tel => telephone */ $telephone = $_POST["telefoonnummer"]; if(!empty( $first_name )){ $post_items['first_name'] = $first_name; } if(!empty( $company )){ $post_items['company_name'] = $company; } if(!empty( $email )){ $post_items['email'] = $email; } if(!empty( $address )){ $post_items['address'] = $address; } if(!empty( $number )){ $post_items['number'] = $number; } if(!empty( $postcode )){ $post_items['postcode'] = $postcode; } if(!empty( $city )){ $post_items['city'] = $city; } if(!empty( $telephone )){ $post_items['telephone'] = $telephone; } if(!empty($postcode) && !empty($number)) { $ch = curl_init(); if ( curl_error($ch) != "" ) { return; } $post_string = json_encode($post_items); $con_url = 'valid api url'; curl_setopt($ch, CURLOPT_URL, $con_url); curl_setopt($ch, CURLOPT_HTTPHEADER, array( "Content-Type: application/json", "Authorization: Token XXX (censored)" )); curl_setopt( $ch, CURLOPT_POSTFIELDS, $post_string); curl_setopt( $ch, CURLOPT_RETURNTRANSFER, true ); $output = curl_exec($ch); file_put_contents("curlerror.txt", $output); curl_close($ch); } return $output; } ```