Drone suddenly flipped and crashed in Stabilize mode..reading LOG after crash

My drone is china f450 with apm 2.6 and 1000kv motors. I’ve been struggling to tune it last week and today I finnaly got some stable results but close to end of my testing the drone just flipped in Stabilize mode and broke 1 arm and propeller.
Is getting very frustrating after finnaly tuned it to just flip and crash by itself.
Please if anyone can discover some good information in my flight log or tell me how to check what was wrong. I’m new with APM trying to figure it out.

I uploaded .bin log here https://files.fm/u/855833ph

Auto analyze restults :

Log File C:\Users\Underground\Documents\Mission Planner\logs\QUADROTOR\1\2017-11-07 14-57-39.log
Size (kb) 2692.1513671875
No of lines 36721
Duration -1 day, 23:13:19
Vehicletype ArduCopter
Firmware Version V3.2.1
Firmware Hash 36b405fb
Hardware Type
Free Mem 0
Skipped Lines 0
Test: Autotune = UNKNOWN - No ATUN log data
Test: Brownout = FAIL - Truncated Log? Ends while armed at altitude 8.54m
Test: Compass = GOOD - No MAG data, unable to test mag_field interference

Test: Dupe Log Data = GOOD -
Test: Empty = GOOD -
Test: Event/Failsafe = FAIL - ERR found: CRASH
Test: GPS = GOOD -
Test: IMU Mismatch = UNKNOWN - No IMU log data
Test: Motor Balance = FAIL - Motor channel averages = [1380, 1365, 1581, 1586]
Average motor output = 1478
Difference between min and max motor averages = 221
Test: NaNs = GOOD -
Test: OpticalFlow = FAIL - ‘FLOW_FXSCALER’ not found
Test: Parameters = GOOD -
Test: PM = GOOD -
Test: Pitch/Roll = GOOD -
Test: Thrust = GOOD -
Test: VCC = UNKNOWN - No CURR log data

I calibrated the motors before flying I don’t know why they still unbalanced. Can that be the crash reason ?

Thanks I appreciate your help.

why so old firmware, and why so little logging ? - maybe very old autopilot ?
shortly before the log ends, there are signs of one of the motors/esc/props failing to perform, then work ok for a short while… then the log end.

Thanks for answer…I have no idea how old is it, ordered it bundle with drone from china…didn’t know about the firmware stuff when I bought the APM and it seems mine is “retired” and this is latest version that works or this is what mission planner says…
What do you mean by little ? like little informations in log or short time log ?
Next day after crash I changed broken arm and tried to test to hover little and one of the motors was just trying to spin while others were spinning while armed. I reconnected lipo and then all good…made this log :

yes, it’s an APM , befor Pixhawk-era. , so logging capacity is not quite there (less information)

They’re very unbalanced.Maybe an accelerometer calibration would help ? You can see one pair of motors running far below the other pair in the RCOUT graph (once you’ve zoomed right in).Just before the crash one of the motors (I forget - green line) dropped to zero and the opposite motor went to max (actually it may be the other way around).Sky/ground/sky/ground/sky/ground/ground.Not sure if this is bad motors or bad balance.You’d have to check the thrust for each motor.And any bullet connectors you use.