Copter critical EKF failure and crash 4.3.7

A few days ago we experienced a critical failure of the drone EKF navigation, while flying with our vision-based navigation capability. We directly tried to switch back to GPS-based, but the drone began to behave erratically, and reported repeatedly EKF failsafe and tried a few time to realign yaw.

Usually, when switching the EKF src back to GPS all the bad behavior fix itself after a few seconds, but this time the drone didn’t recover and all position-based modes failed, and the pilot didn’t succeed in taking effective control in alt hold until the battery got loose after the erratic flight (causing a crash from high altitude which mercifully ended with negligible damages).

From what I get from the logs, it seems that somehow the EKF failed to estimate correctly direction, up to a point after a yaw re-alignment with GPS that its yaw was inversed by 180 deg.

Here is a link to the log file, which by chance was writing extended EKF data:
Link to drive