Add own backend interface to the logger failed

Question

I try to send the NRF_LOG_INFO() messages over a own backend interface. I tried to add a new backend with nrf_log_backend_add() and enable it with nrf_log_backend_enable(). But as soon as I do this, I only get the first log print over RTT and afterwards I receive nothing. I seems the software is "stuck" somewhere. 
 NRF52832, SDK 15.3 and Softdevice 6.1.1. #define NRF_LOG_BACKEND_RTT_ENABLED 1 in sdk_conf.h 
 void logPut(nrf_log_backend_t const * p_backend, nrf_log_entry_t * p_msg)
{
 //send over uart
}

static void logFlush(nrf_log_backend_t const * p_backend)
{
 //send over uart
}

static void logPanic(nrf_log_backend_t const * p_backend)
{
 //send over uart
}

const nrf_log_backend_api_t sOwnLogBackendApi = {
 .put = logPut,
 .flush = logFlush,
 .panic_set = logPanic,
};

static nrf_log_backend_t sOwnLogBackend = {
 .p_api = & sOwnLogBackendApi,
};

void logInit()
{
 const ret_code_t err_code = NRF_LOG_INIT(NULL);
 APP_ERROR_CHECK(err_code);

 NRF_LOG_DEFAULT_BACKENDS_INIT();

 int32_t backend_id = nrf_log_backend_add(& sOwnLogBackend, NRF_LOG_SEVERITY_DEBUG);
 APP_ERROR_CHECK_BOOL(backend_id >= 0);
 nrf_log_backend_enable(& sOwnLogBackend);
}

void uartInit(uartMessageHandler_t uartMessageHandler)
{
 sUartMessageHandler = uartMessageHandler;

 const app_uart_comm_params_t comm_params =
 {
 .rx_pin_no = RX_PIN_NUMBER,
 .tx_pin_no = TX_PIN_NUMBER,
 .rts_pin_no = RTS_PIN_NUMBER,
 .cts_pin_no = CTS_PIN_NUMBER,
 .flow_control = APP_UART_FLOW_CONTROL_DISABLED,
 .use_parity = false,
 .baud_rate = NRF_UART_BAUDRATE_115200
 };

 uint32_t result;
 APP_UART_FIFO_INIT(& comm_params,
 UART_RX_BUF_SIZE,
 UART_TX_BUF_SIZE,
 onUartEvent,
 APP_IRQ_PRIORITY_LOWEST,
 result);
 APP_ERROR_CHECK(result);
}