Hi there! 
I couldn’t find anything else regarding my issue…
I just came across a problem. My horizon is drifting slightly. While arming, I have a yellow flashing led (pre-arm error). I opened MP, and counted out every element, which is listed on prearm list. It turned out, that it has no gps lock (doing it indoors - normal), and INS are not calibrated (however this wasn’t flashed on the HUD, and I have calibrated accel many times). I am not using any external compass (plane), Tried to calibrate inner compass (PX4), and it threw out compass 1 error: 99 (same for compass 2, and 3), but it started to collect samples, and in the end I got information, that calibration ended with success.
Trying to arm the plane - everything goes round again - it doesn’t want to arm, yellow led, etc…
If I deselect GPS lock, and INS - it works. The led blinks blue (no gps lock), and PX arms, but what is wrong with the INS?
Did anyone have to deal with that kind of problem?
Best regards,
Kuba