Getting "Unhandled api error" in PayPal integration

Currently running a sandbox integration of PayPal Buttons. After clicking on PayPal Button, the createOrder method in:

paypal.Buttons({
    createOrder: function() {
        return fetch('php/registerPayPalPayment.php', {
            method: 'post',
            body: JSON.stringify({orderID: <?php echo $orderID; ?>}),
            headers: {
                'content-type': 'application/json'
            }
        }).then(function (res) {
            console.log("Paypal: CreateOrder");
            console.log(res);
            return res.json();
        }).then(function (data) {
            console.log(data);
            return data.id; // Use the same key name for order ID on the client and server
        });
    }

returns a successfull response:

{id: "32509608BX945762G", links: Array(4), status: "CREATED"}

After log in into PayPal Sandbox account. An error occurrs within the "create" call:

ack: "error"
message: "Unhandled api error"
meta: {calc: "dc72e617fec59",…}
server: "Hkq76r..."

I don´t know where the problem is, it previously worked fine in december 2019.