Alt Hold and Loiter don't takeoff

I set the parameter back to its 250 default.

I was able to takeoff, but after a few seconds the aircraft would slowly go down and not go up again, even with my throttle stick at 100%.

2017-11-29 21-22-05.bin (1.3 MB)

Here is a video: Loiter mode flight attempt - YouTube
Video description: I took off on loiter mode. After a few seconds in the air, the drone started to go down until it landed without my wish. At 0:36 you can hear the motors spin faster: that’s me putting my throttle stick at 100%. Given it didn’t take off again, I gave up and disarmed the motors.

Should I be concerned about the “EKF2 IMU0 fround mag anomaly” on the log file?
Which height measures should I check my log for in order to see if it also shows the drone decended?

I also wanted to try and see if the problem was that my throttle mid lever is not set (my log shows that I hover at a 27% throttle on stabilize). But I didn’t find the variable I should change in order to modify the throttle mid, as I have stated here: Throttle mid too low - #4 by dedezago