I have seen similar issues with systems that are powered 24/7 that really aren't meant to be. If the capacitors for the on-board regulators are not of high quality then over time they swell up and sometimes even leak their guts.

I have a Windows 7 desktop computer that runs 24/7 as my home DVR. It is running the Free Windows Media Center. I have had to replace the motherboard once already because of bulging capacitors. Then the replacement motherboard began to have issues. Since I now knew the problem was capacitors I replaced the ones I found bulging which fixed the problem.

Going back to my failing Z400 the reason it failed was when it got powered down the capacitors had pressure reduced because of no power. The constant pressure then no pressure is what caused it to open up and thus lose it's value causing incorrect power on the motherboard.

Because of this issue you may need more than a few Z400 spare motherboards. If a time comes when you have to power off your Z400 miners a number of them may not power up again without issues.