USB-CDC-driver hangs in USBD_Write

char USBD_Write(unsigned char eptnum,
                const void *data,
                unsigned int size,
                TransferCallback callback,
                void *argument)
{
    Endpoint *endpoint = &(endpoints[eptnum]);
    Transfer *transfer = &(endpoint->transfer);

    // Check that the endpoint is in Idle state
    if (endpoint->state != UDP_ENDPOINT_IDLE) {
        return USBD_STATUS_LOCKED;
    }

    trace_LOG(trace_INFO, "Write%d(%u) ", eptnum, size);

    // Setup the transfer descriptor
    transfer->data = (char *) data;
    transfer->remaining = size;
    transfer->buffered = 0;
    transfer->transferred = 0;
    transfer->callback = callback;
    transfer->argument = argument;

    // Send the first packet
    endpoint->state = UDP_ENDPOINT_SENDING;
    while((AT91C_BASE_UDP->UDP_CSR[eptnum]&AT91C_UDP_TXPKTRDY)==AT91C_UDP_TXPKTRDY);
    UDP_WritePayload(eptnum);
    SET_CSR(eptnum, AT91C_UDP_TXPKTRDY);

    // If double buffering is enabled and there is data remaining,
    // prepare another packet
    if ((BOARD_USB_ENDPOINTS_BANKS(eptnum) > 1) && (transfer->remaining > 0)) {

        UDP_WritePayload(eptnum);
    }

    // Enable interrupt on endpoint
    AT91C_BASE_UDP->UDP_IER = 1 << eptnum;

    return USBD_STATUS_SUCCESS;
}