Crash in guided flight: Radio Failsafe - Disarming

ArduCopter 4.0.3 on pixracer, hexa, managed with companion computer in GUIDED mode, rc reciver (X8R) disconnected and removed from the drone after successful autotune.
Several times everything flew without problems. Yesterday, after 20 minutes of flight, motors stops in the flight, at this moment drone was at alt about 20 meters. Before the ground, motors started again, but the drone completely crashed and destroyed. flying was in GUIDED mode, mostly it hover over the point, no active movements.
I have no bin logs for this flight, but only messages from aurdupilot in my log.
18:36:16,727 STATUSTEXT {severity : 7, text : RCInput: decoding PPM}
18:36:16,732 STATUSTEXT {severity : 4, text : Radio Failsafe Cleared}
18:36:17,253 STATUSTEXT {severity : 4, text : Radio Failsafe}
18:36:17,540 STATUSTEXT {severity : 4, text : Radio Failsafe Cleared}
18:36:17,971 STATUSTEXT {severity : 4, text : Radio Failsafe - Disarming}
18:36:18,025 STATUSTEXT {severity : 4, text : Radio Failsafe Cleared}

“Radio Failsafe - Disarming” and “Radio Failsafe Cleared” logged many times until
at 18:37:02,387 I got “No heartbeat in 30 seconds” exception and drone was already crushed at this time.

“RCInput: decoding PPM” is too strange for me, because we use sbus connection at x8r. 30 secs is ok for pixracer reboot, but if this occured, why motors started again since nobody submit new command to ardupilot, usually starting up require at list few minutes.

All hardware was checked before fly and there no problem with it, this drone was flying well without problem. esc and motors is ok, there are no overheat or motor sync missing.
fiilsafe params:

I would be grateful for any ideas why it occurred and how to solve?

We really, really need a .bin log file.

Well, there are probably two problems. The first is that radio failsafe
is being entered; that’s bad and you should probably check your preivous
logs to see if the same thing was happening.

The second is that your vehicle thought it was landed - when it wasn’t.
If we get a radio failsafe when the vehicle is landed we disarm instantly.

That can happen if a vehicle is suffering excessive vibrations. You can
check the RAW_IMU messages in your telemetry log to see if that is the

Dataflash logs are important for diagnosing things post-crash. If you
want to get value-for-money out of your crashes, you should look at
getting that working.