I've now verified the bug in UA's systems and updated the Wiki at the top accordingly. I can reliably reproduce this error: If you book a Miles-based ticket but do not have enough Miles to complete the booking immediately (eg you intend to xfer in Miles from Chase or whatever), then the attempt to complete the FareLock will fail with a generic error message.

Agents don't understand this either. They'll try the same things (and it will take a LLLLLOOOOOONNNGGG time) to finally do nothing. Elite MP members may get an agent who will put a reservation on a 24-hr or other hold, but that's not guaranteed.