So I was recently plagued by the "kernel: dcd/xxxx: potentially unexpected fatal signal 6" error. Did some searching and saw it was a bug in 12_3. Went through the normal upgrade steps via the HTTP interface which worked great on my RT-AC68U but then I got a suspicious "Please manually reboot" at the end of the RT-AC5300 flash. Sadly it never came back. Tried a hard reset numerous times to no avail. Finally, I tried rescue mode this morning (probably should have gone straight there) using the 12_4 image. Everything is happy again. Guess it was just gremlins in the ether.