I have small company with a one year old Dell server on a 1400 W UPS that will intermittently, approximately once a month, reboot unexpectedly. There is no hint in any of the Event Viewer catagories, jut the Unexpected Reboot error as the server comes back up.
I guessing this is a hardware issue, probably server motherboard, but there are many other possibilities: Dual Power Supply +/or Controller, UPS output failing even with normal constant AC power on its input, something shorting out and dragging the power supply way down momentarily, etc. There is nothing unusual connected to the server, just LCD monitor, wired keyboard and wired optical mouse, one Ethernet, and one External USB Hard Drive for direct backup. Only the Server, Monitor and EHD are on the UPS.
I am also wondering if this could be some kind of software issue: OS, UPS software, patch, or virus, although Bit Defender has been pretty good.