The Adidas support did a great job, I was told to follow this instruction to downgrade the firmware and restore my miCoach Pacer to factory setting. Too bad there are too many steps there and my Pacer didn't behave exactly as described. Support gave me another text file which can be launched as commands in Terminal to reformat the pacer which is no different from an ordinary external storage device. It worked.
I wonder if the software logs user actions (such as what buttons are pressed, switch location etc). The good thing about having the miCoach Manager to do the reformatting (instead of that in the forum Q&A, involving too many steps, easy to make mistake and misunderstanding) is that the desktop app can retrieve error logs, perform diagnostic, send them to miCoach support, before reformatting is done.