"check your firmware version on the APM, 3.3 isn't supported on a 2.6 - last supported version is 3.2 (I'm running 3.2.1 and it seems OK)

I had to reset my APM using mission planner after I realised I couldn't use 3.3 and reset it to 3.2.1

cheers!"

"I just had the same problem - flipped forward in hexa X config, flipped backwards in Hexa +
APM 2.8
ublox 6M GPS  and compass

I got it working by:
installing v3.2.1 of copter (last version the APM board supports)
resetting the APM board to factory…"