Creating a bill returns code 1000 "Internal Error" with API v3

Hi all, I'm creating a script who creates a new bill in the admin panel via API v3 and PHP 7 ( https://www.zoho.eu/books/api/v3/#Bills_Create_a_bill )

This is a testing array I'm sending to the API

$data = array(
"vendor_id" => ZOHO_VENDOR_ID ,
"bill_number" => "1234" ,
"date" => "2019-04-01" ,
"line_items" => array( array(
"quantity" => "1" ,
"name" => "Testing Bill Name" ,
"rate" => "1000" ,
"account_id" => ZOHO_ACCOUNT_ID ,
"tax_name" => "Zero VAT Rate [0%]" ,
"tax_percentage" => 0
) ) ,
"notes" => "notes123"
);
$curlData = "JSONString=" . urlencode( json_encode( $data ) ) . "";
/* JSON GENERATED
JSONString=%7B%22vendor_id%22%3A%22<ZOHO_VENDOR_ID>%22%2C%22bill_number%22%3A%221234%22%2C%22date%22%3A%222019-04-01%22%2C%22line_items%22%3A%5B%7B%22quantity%22%3A%221%22%2C%22name%22%3A%22Testing+Bill+Name%22%2C%22rate%22%3A%221000%22%2C%22account_id%22%3A%22<ZOHO_ACCOUNT_ID>%22%2C%22tax_name%22%3A%22Zero+VAT+Rate+%5B0%25%5D%22%2C%22tax_percentage%22%3A0%7D%5D%2C%22notes%22%3A%22notes123%22%7D
*/
$ch = curl_init();
curl_setopt( $ch , CURLOPT_URL ,"https://books.zoho.eu/api/v3/bills?organization_id=" . ZOHO_ORGANIZATION_ID );
curl_setopt( $ch , CURLOPT_POST , true );
curl_setopt( $ch , CURLOPT_HTTPHEADER , array(
'Authorization: Zoho-authtoken ' . $access_token,
'Content-Type: application/x-www-form-urlencoded;charset=UTF-8'
));
curl_setopt( $ch , CURLOPT_POSTFIELDS , $curlData );
curl_setopt( $ch , CURLOPT_RETURNTRANSFER , true );
$server_output = curl_exec( $ch );
$info = curl_getinfo( $ch );
curl_close ( $ch );
var_dump( $server_output );

But the response from the API is

'{"code":1000,"message":"Internal Error"}' (length=40)

I tried different edits with the $curlData variable:

1. urlencode + json_encode ( without JSON_PRETTY_PRINT )

$curlData = array( "JSONString" => urlencode( json_encode( $data ) ) );
// JSON GENERATED
%7B%22vendor_id%22%3A%22<ZOHO_VENDOR_ID>%22%2C%22bill_number%22%3A%221234%22%2C%22date%22%3A%222019-04-01%22%2C%22line_items%22%3A%5B%7B%22quantity%22%3A%221%22%2C%22name%22%3A%22Testing+Bill+Name%22%2C%22rate%22%3A%221000%22%2C%22account_id%22%3A%22<ZOHO_ACCOUNT_ID>%22%2C%22tax_name%22%3A%22Zero+VAT+Rate+%5B0%25%5D%22%2C%22tax_percentage%22%3A0%7D%5D%2C%22notes%22%3A%22notes123%22%7D
// returns

'{"code":4,"message":"Invalid value passed for JSONString"}' (length=58)

2. urlencode + json_encode ( with JSON_PRETTY_PRINT )

$curlData = array( "JSONString" => urlencode( json_encode( $data , JSON_PRETTY_PRINT ) ) );
// JSON GENERATED
%7B%0A++++%22vendor_id%22%3A+%22<ZOHO_VENDOR_ID>%22%2C%0A++++%22bill_number%22%3A+%221234%22%2C%0A++++%22date%22%3A+%222019-04-01%22%2C%0A++++%22line_items%22%3A+%5B%0A++++++++%7B%0A++++++++++++%22quantity%22%3A+%221%22%2C%0A++++++++++++%22name%22%3A+%22Testing+Bill+Name%22%2C%0A++++++++++++%22rate%22%3A+%221000%22%2C%0A++++++++++++%22account_id%22%3A+%22<ZOHO_ACCOUNT_ID>%22%2C%0A++++++++++++%22tax_name%22%3A+%22Zero+VAT+Rate+%5B0%25%5D%22%2C%0A++++++++++++%22tax_percentage%22%3A+0%0A++++++++%7D%0A++++%5D%2C%0A++++%22notes%22%3A+%22notes123%22%0A%7D
// returns
'{"code":4,"message":"Invalid value passed for JSONString"}' (length=58)

3. json_encode ( without JSON_PRETTY_PRINT )

$curlData = array( "JSONString" => json_encode( $data ) );
// JSON GENERATED
{"vendor_id":"<ZOHO_VENDOR_ID>","bill_number":"1234","date":"2019-04-01","line_items":[{"quantity":"1","name":"Testing Bill Name","rate":"1000","account_id":"<ZOHO_ACCOUNT_ID>","tax_name":"Zero VAT Rate [0%]","tax_percentage":0}],"notes":"notes123"}
// returns

'{"code":4,"message":"Invalid value passed for JSONString"}'


    
     

    
    
     4. json_encode ( with JSON_PRETTY_PRINT )
     

    
    
     

    
    
     
      $curlData = array( "JSONString" => json_encode( $data , JSON_PRETTY_PRINT ) );

      // JSON GENERATED

      {
    "vendor_id": "<ZOHO_VENDOR_ID>",
    "bill_number": "1234",
    "date": "2019-04-01",
    "line_items": [
        {
            "quantity": "1",
            "name": "Testing Bill Name",
            "rate": "1000",
            "account_id": "<ZOHO_ACCOUNT_ID>",
            "tax_name": "Zero VAT Rate [0%]",
            "tax_percentage": 0
        }
    ],
    "notes": "notes123"
}

      // returns

      '{"code":4,"message":"Invalid value passed for JSONString"}' (length=58)

Is there any error in my request or JSON encoding? Thank you for your replies.