I had the exact same problem. In the end, I re-imaged my flash drive from a backup. That still didn't work. Then after rebooting the server around 10 total times, it eventually started working randomly. That was about 2 months ago, and it seems to be continuing to function...
I don't really have anything too helpful to add, sadly. My theory is that something just doesn't work right when the server reboots from a UPS-driven shutdown. As I said, what happened to you was pretty much identical to my experience.