When I use NRF_LOG_GETCHAR() witn nRF52 DK and onboard SEGGER UART it stops in next NRF_LOG_INFO(). Also after ARM reset it keeps in this error state. It seems that SEGGER does not correctly drive CTS pin.
Also it is strange that when I disable UART flow control setting:
#define NRF_LOG_BACKEND_SERIAL_UART_FLOW_CONTROL 0
Then NRF_LOG_GETCHAR() does not work. But at least NRF_LOG_INFO() works after ARM reset.