Dear all,
please take a look at this conversation Link
Seems that several people have problems with getting (more or less) random prearm failure messages on different Pixhawk hardware implementations using 3.3 firmware. I personally have a 3DR one and another one from rcmaster both showing the same behavior which for me is a strong indication that this is not hardware dependent. I used both Pixhawks also with older firmwares and can not remember such kind of behavior.
Let me know if you need detailed information/logs so i will try to send these here.
Thanks for taking this up!
Thomas