EKF is "Extended Kalman Filter", a SW part of the flight controller FW to read all sensor data and compute the attitude of the drone (position, direction, acceleration and angles in in all 3 dimensions).
With the above very rare information we can only guess. Maybe a sensor problem, a FW problem or to much vibrations due to defect propeller and so and on...
Maybe more information we could find in the flight log from the controller device. I do not know, if one can download also logs from the drone itself (aka ULOG files).
br HE