This log message seems to be generated in the case of a BT_CONN_DISCONNECT_COMPLETE when the reason for disconnection is "BT_HCI_ERR_CONN_FAIL_TO_ESTAB". So upon a disconnection, what does your application do? It should be fine to just restart advertising/scanning in order to reconnect. Also, what version of the nRF Connect SDK are you using for development here, and do you see any kind of logging on the device it tries to connect to when this occurs?
