PHP Paypal MALFORMED_REQUEST_JSON although JSON seems to be well formatted

I try to implement a paypal express checkout flow to a website.

The user must be able to adjust the order after they have logged in with paypal and choosed their payment option.

To do so, I create the order with paypal with the intent “AUTHORIZE” and user action “CONTINUE”. It is created, the user is sent back to our website and I can fetch order and payer information with the (order) id created.

But then, when the payer is done checking and adjusting some final parameters and clicks the “buy now” button, I want to send the update order call as referenced here:
https://developer.paypal.com/api/orders/v2/#orders_patch

I create the payload in exactly the same way as for order creation, but this time with intent “CAPTURE” and user_action=”PAY_NOW”. I PATCH it to the correct path, but I keep getting the failure message “MALFORMED_REQUEST_JSON” .

Hoewever, the JSON itself is a valid JSON, no errors thrown in creation. Here is a sample:

{
    intent: "CAPTURE",
    application_context: {
        landing_page: "NO_PREFERENCE",
        shipping_preference: "SET_PROVIDED_ADDRESS",
        user_action: "PAY_NOW",
    },
    purchase_units: [{
        reference_id: "2289256",
        description: "Your Order with us",
        custom_id: "OrderId 2289256",
        soft_descriptor: "Name of the site",
        invoice_id: "2289256",
        amount: {
            currency_code: "EUR",
            value: 59.98,
            breakdown: {
                item_total: {
                    currency_code: "EUR",
                    value: 50.41,
                },
                shipping: {
                    currency_code: "EUR",
                    value: 0,
                },
                discount: {
                    currency_code: "EUR",
                    value: 0,
                },
                tax_total: {
                    currency_code: "EUR",
                    value: 9.57,
                },
            },
        },
        items: [{
            name: "Product 1",
            description: "Product 1 description",
            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 2 description",
            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(Berlin)",
                postal_code: "10715",
                country_code: "DE",
            },
        },
    }],
}

Here is paypal´s response

{
    name: "INVALID_REQUEST",
    message: "Request is not well-formed, syntactically incorrect, or violates schema.",
    debug_id: "c315ce9eb90b4",
    details: [{
        field: "/",
        location: "body",
        issue: "MALFORMED_REQUEST_JSON",
        description: "The request JSON is not well formed.",
    }],
    links: [{
        href: "https://developer.paypal.com/docs/api/orders/v2/#error-MALFORMED_REQUEST_JSON",
        rel: "information_link",
        encType: "application/json",
    }],
}

I just cannot figure out the problem. I tried to remove the whole application_context, purchase_units, intent and see if there is a problem within any of these parameters. Nothing changed. What exactly is wrong with this call?