I am having the same issue as posted under this link with a cube black running 4.0.5 . I thought is was a problem running blended CAN configured Here 2 gps/compasses but I continue to get prearm IMU mismatch errors for about 3-4 minutes then if I reboot I get a clean prearm immediately with no mismatch. The problem only occurs on cold boot once it warms up for about 3-4 minutes I have no problems. This is very consistent. I tried the above method and disabled IMU2 and then rebooted cold and the problem does not exists. It booted up and immediately I got a clean prearm. Any help with this is greatly appreciated.