STM32 Server (lwip) response to Javascript (fetch-API)

Question

I have the following project: I want to send data from a web browser to an STM32H743 microcontroller via Javascript. I am using the lwip stack on the microcontroller. If I now send data to my STM server with the following Javascript code, I receive the sent string with the additional HTTP add-ons on my microcontroller.

const serverAddress = "http://192.168.1.87:8000";
function sendDataToServer(data) {
    console.log("Sending data to the server.");
    fetch(serverAddress, {
        method: 'POST',
        headers: {
            'Content-Type': 'text/plain'
        },
        body: data
    })
    .then(response => {
        console.log(response);
        if (!response.ok) {
            throw new Error('Network response was not ok.');
        }
        console.log('Data successfully sent to the server.');
    })
    .catch(error => {
        console.error('Error when sending data to the server:', error);
    });
}

// Function call from a button-event
sendDataToServer('hello server!');

The received string on the STM32:
"POST / HTTP/1.1\r\nHost: 192.168.1.87:8000\r\nConnection: keep-alive\r\nContent-Length: 12\r\nUser-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) ... US;q=0.8,en;q=0.7\r\n\r\nhello server!"

With the following callback function I send the string response

HTTP/1.1 200 OK\r\n
Content-Length: 0\r\n\r\n

with the function tcp_write() back to the client, as shown below.

void serverReceiveCallback(void *arg, struct tcp_pcb *tpcb, struct pbuf *p, err_t err) {
    if (p != NULL && err == ERR_OK) {
        // Copy the received Data in a local buffer.
        char rxBuf[rxBufferSize];
        memcpy(rxBuf, p->payload, p->len);
        // Terminate the string.
        rxBuf[p->len] = '\0';
        // Is the received String a post?
        if (p->len > 0) {
            if (strncmp(rxBuf, "POST", 4) == 0) {
                // Respond to the post.
                const char response[] = "HTTP/1.1 200 OK\r\nContent-Length: 0\r\n\r\n"; // SENT
                tcp_write(tpcb, response, strlen(response), 0);
            }
        }
        // Free the pbufs
        tcp_recved(tpcb, p->tot_len);
        pbuf_free(p);
    }
}

Since I am working with the lwip package on the transport layer, I have to do the header for the HTTP protocol myself and obviously I am doing something wrong there. The browser then tells me:

net::ERR_INVALID_HTTP_RESPONSE

Do I have to design the header differently so that the fetch API of Javascript is satisfied?

Many thanks for your help!

Touby