What is failsafe short event on?

Hi ,
I have a strange failsafe that rc4 suddenly have no signal


It looks that trigger the fail safe event, but I dont’t know the failsafe reason?
does anyone could help to analysis the log?
the flight log is below:
https://drive.google.com/file/d/1PnNGwSAyDVjFwjzt7Tqs_1LwUyh8wrGT/view?usp=sharing