I can get out a volt meter and do some checking. Due to poor electrical design, the garage has one circuit that powers a freezer, the central vacuum, and all my techie equipment. It's a 20 amp circuit, but is technically near capacity.
I had assumed that voltage was not a problem in the past, because operation of the central vacuum, a heavy beast with a big motor, doesn't disrupt anything. If voltage were low on this circuit, running the central vac would cause enough voltage drop to trigger the UPS, but it never has. (Perhaps, though, the central vac has taken a toll on the UPS electronics.)
Running a new power circuit to my techie equipment is an expensive option, but could be considered. We've been in this place 7 years without the central vac causing a problem, so I hope I don't have to do something now. But....
The mystery remains about AM.6.40. I can't think of anything on a timer that could be causing this. Perhaps its a load in a neighbor's house, though again, why would some remote load cause a voltage drop that my central vac isn't capable of doing?
Perhaps the UPS itself has been overloaded. It is a large-capacity unit, but I don't recall the rating now. Plugged into it are the Partner ACS and the Windows server, plus cable modem and router. Perhaps the ACS, doing its morning voice mail maintenance, is causing a temporary power suck, and yanking the UPS down. That might indicate a slightly defective power supply in the ACS and/or UPS failure. I'll learn more when I replace the UPS.