Paypal Integration in PHP checkout/v2/UpdateOrder REFERENCE_ID_NOT_FOUND

I try to implement paypal express checkout in a way that users are able to manipulate their order after they logged in with Paypal and when they are done, click “bux now” and the actual payment is capured.

To do so, I created the order with paypal before with intend “Authorize”. The user logs into paypal and comes back to my shop to manage the order. When done, I want to use paypals updateOrder Call to apply the users changes. But It always bounces with error REFERENCE_ID_NOT_FOUND.

Here is the documentation:
https://developer.paypal.com/api/orders/v2/#orders_patch

Here is, what I send to Paypal.

[{
    "op": "replace",
    "path": "/purchase_units/@reference_id=='default'",
    "value": [{
        "reference_id": "2295037",
        "description": "Your order at Shop",
        "custom_id": "Shop Id 2295037",
        "soft_descriptor": "Shopname",
        "invoice_id": "2295037",
        "amount": {
            "currency_code": "EUR",
            "value": 59.98,
            "breakdown": {
                "item_total": {
                    "currency_code": "EUR",
                    "value": 50.4
                },
                "shipping": {
                    "currency_code": "EUR",
                    "value": 0
                },
                "discount": {
                    "currency_code": "EUR",
                    "value": 0
                },
                "tax_total": {
                    "currency_code": "EUR",
                    "value": 9.58
                }
            }
        },
        "items": [{
            "name": "Product 1",
            "description": "Product Descr. 1",
            "sku": "1019879",
            "unit_amount": {
                "currency_code": "EUR",
                "value": 16.8
            },
            "tax": {
                "currency_code": "EUR",
                "value": 3.19
            },
            "quantity": "1",
            "category": "PHYSICAL_GOODS"
        }, {
            "name": "Product 2",
            "description": "Product Descr. 2",
            "sku": "1024593",
            "unit_amount": {
                "currency_code": "EUR",
                "value": 33.61
            },
            "tax": {
                "currency_code": "EUR",
                "value": 6.38
            },
            "quantity": "1",
            "category": "PHYSICAL_GOODS"
        }],
        "shipping": {
            "name": {
                "full_name": "John Doe"
            },
            "address": {
                "address_line_1": "Badensche Str. 24",
                "address_line_2": "",
                "admin_area_2": "Berlin",
                "postal_code": "10715",
                "country_code": "DE"
            }
        }
    }]
}]

The answer from Paypal is “REFERENCE_ID_NOT_FOUND”. Paypal itself suggests ‘default’ if there is only one purchase unit (which it is). I also tried to replace ‘default’ with the reference_id in our system (which was provided with the createOrder call) or paypal´s own transactionId (which is provided through the endpoint anyway). The error is always the same.

How can I change this call so that Paypal accepts it and updates the order in its system?