The latest sticking point was Windows 10 Build 16215, which would endlessly reboot, fail to upgrade, and revert. I was on the Fast Ring, trying to upgrade to Build 16226–even though more recent Windows 10 builds were available. After waiting some days, I put the PC to Slow Ring, whereupon Windows Update said I was up to date. When Build 16232 was released to Slow Ring, the upgrade was successful.
So in summary, to stop the upgrade loop, drop back to Slow Ring, wait for the next Slow Ring build, then hop back onto Fast Ring. There may be more Forceful ways to do this, but I prefer avoiding the risk of losing the system configuration.