This thread seems to have died. Was OP ever able to find out what caused the calibration issue ?
I'm on my 2nd new MPP (1st defective, couldn't be activated) in 2 weeks and now it won't calibrate no matter how many times or places that I try. I don't wear a watch and the only metal on or near me when I fly is my titanium eye glass frames, my gold ring, an artificial joint, phone and the RC. I keep my Galaxy S7 in airplane mode when using it with the MPP. I'm running the latest versions of all firmware and GO 4. I've experienced random calibration errors and lost GPS with as many as 16 satellites both before and during flights on both birds. I've calibrated the IMU and compass repeatedly in an effort to remedy these issues. I'm generally flying line of sight at an altitude of 100-400 ft. on a large multi-acre, flat terrain parcel with no power lines or metal structures. Now when I check the sensor status for Compass, the #1 shows a solid red bar. Is this an indication that I'm just unlucky and the compass has failed ?