Octa X crash in stabilize

I’m working on getting an octocopter flying, and after a few good flights the day before and a controlled flight into a bush (we found no damage) followed by an aborted autotune attempt (We didn’t expect such large attitude changes and I switched back to stabilize and landed with no problems), we took off again to get some practice with manual flight control, and after ascending to 4 or 5 meters altitude and starting to back off to hold altitude, the drone began a rapid descent and had some issues holding orientation.
It seems like a rotor shutdown would explain it but afaik we should be able to fly with one rotor out. There weren’t any problems with power failure or motor desyncs when we did some full power climb rate tests, and we have logs all the way until we shut down (unfortunately not much video).

-Octocopter X with BeagleBone Blue running Copter 3.5.1
-XOAR T5010 motors with 50a BLHeli_32 ESCs (Acceleration limited to 0.5%/ms)
-19.5x7 XOAR CF props
-M8N GPS (Reports bad compass health so we’ve been using the compass on the BeagleBone, which reports interference sometimes but didn’t cause a crash or instability during high speed climbs)
-4x custom built NCR20700-based batteries, wired for 6S5P each giving 20Ah/pack with 4C discharge
-All-up weight roughly 25-30lbs

AltHold works fine, PosHold kinda works but needs tuning and probably a working external compass. I think the FC is working fine but I’m not really sure, we haven’t found a definitive cause in the logs but don’t have a huge amount of experience in what to look for (the hexa we built on an F550 frame with another BeagleBone Blue didn’t have this issue). Motors and ESCs don’t have visible damage, we’ll get to test them all for function tomorrow. The only damage was that some of the CF arms were damaged by the landing leg clamps.


Here’s the only video we have of the crash, shows the last second or so of flight: https://drive.google.com/open?id=1QDuOrrrBUQBnkGzpbHJfuKTm9t30xqgy

Video of operation the day before showing (commanded) full power climb, with some difficulties due to improperly calibrated level horizon (fixed before the next day): https://drive.google.com/open?id=1Dgctv_t9YZ4wcygcQzd1R9oOd7oOXYlA