PayPal Smart Payment Buttons - Alternative Payment Methods - 400 Bad Request - paypal

Question

I'm using the PayPal Smart Payment Buttons on my website to capture payments.
The Order is created server-side via the PayPal-checkout-sdk for PHP.
$request = new OrdersCreateRequest();
$request->prefer('return=representation');
$request->body = self::buildRequestBody($paymentAC);
// 3. Call PayPal to set up a transaction
$client = PayPalClient::client();
$response = $client->execute($request);
The request body looks like this:
return array(
'intent' => 'CAPTURE',
'commit' => true,
'application_context' =>
array(
'brand_name' => 'EXAMPLE INC',
'locale' => 'de-DE'
'shipping_preference' => 'NO_SHIPPING'
),
'purchase_units' =>
array(
0 =>
array(
'reference_id' => $rId,
'description' => 'Ihr kauf bei EXAMPLE INC',
'invoice_id' => $paymentAC,
'amount' =>
array(
'currency_code' => $currency, #EUR
'value' => $price,
'breakdown' =>
array(
'item_total' =>
array(
'currency_code' => $currency,
'value' => $itemTotal,
),
'tax_total' =>
array(
'currency_code' => $currency,
'value' => 0,
),
),
),
'items' => $items,
),
),
);
The request works fine and an order is created. The script returns a JSON Object with the "id" key set to the ID of the created order.
{"id": "PAYMENTID"}
The script is called by the payment button like this:
paypal.Buttons({
createOrder: function() {
return fetch('./CreateOrder.php', {
method: 'post',
headers: {
'content-type': 'application/json'
},
body: '{"payment_ac": "<?php echo $_GET['code'] ?>"}'
}).then(function(res) {
console.log(res);
return res.json();
}).then(function(data) {
return data.id; // Use the key sent by your server's response, ex. 'id' or 'token'
});
},
onError: (err) => {
console.error('error from the onError callback', err);
},
onApprove: function(data, actions) {
// This function captures the funds from the transaction.
$("#loadMe").modal({
backdrop: "static", //remove ability to close modal with click
keyboard: false, //remove option to close with keyboard
show: true //Display loader!
});
return actions.order.capture().then(function(details) {
// This function shows a transaction success message to your buyer.
console.log(details);
//window.location.href = "verify.php?id=" + data.orderID + "&ac=<?php echo $internalID ?>";
$.get({
url: "/payment/verify.php?id=" + data.orderID + "&ac=<?php echo $internalID ?>",
success: function (respdata) {
if (respdata['payment'] === "completed") {
window.location.href = '/success/<?php echo $internalID?>&cid=' + respdata['cid'];
} else {
alert(respdata);
}
}
})
});
},
onCancel: function (data) {
console.log(data);
alert(data);
// Show a cancel page, or return to cart
window.location.href = "/payment/cancelled.php?ac=<?php echo $internalID ?>&oid=" + data.orderID;
},
style: {
color: 'black',
label: 'pay',
layout: 'vertical'
}
}).render('#paypal-button-container');
The payment buttons can open the checkout window successfully.
When I'm using sandbox credentials everything works fine. (PayPal, Simulated Alternative Payments & Credit Card payments)
When I'm switching to live credentials only PayPal payments seem to work. Every alternative payment method redirects to https://www.paypal.com/latinumcheckout/error/generic-error after you enter your details on https://www.paypal.com/latinumcheckout/change-userinfo
When in the developer console there is an error output:
the server responded with a status of 400 () app.bundle.js:41:140543
Credit card payments seem to fail as well.
I'm clueless, I spent a long time reading the documentation but have found no solution.