Why does the server restart suddenly?

Question

Iron

There are 10 servers in our server room, but very specific things happen to two of them. Almost every Saturday at 3 am they suddenly reboot. According to the logs, everything looks as if the power was turned off, and according to the Last State, it then turns on.
Reinstalling the OS several times, initially installed Windows 2008 R2 SP1. One of them was tested with Windows 2003 Sp2. Of the additional software is DrWeb 8, Java, Firebird fork, Tomcat (in other server software the same set).
Was connected via an offline UPS, now switched to a double conversion UPS, but the problem remains. There are no left tasks in the scheduler. Memory and HDD / SSD have been checked more than once.
There is a third server of the same purchase and the same configuration, only with more memory, installed in the same server room, there are no problems with it.
I can no longer imagine which way to dig, given that this is happening as scheduled.

Answer 1

Maybe a completely idiotic assumption, but this is not an update service?

Answer 2

Also, as a not very realistic option: it may be written in the BIOS to turn off, or somewhere a service has been raised that turns off according to a schedule via AMT / vPro.

Answer 3

Or maybe the electricity is simply turned off for a long time, or does it simply sag below the UPS threshold? Is there any UPS monitoring software installed? Set up logs in it and see what happens.

Answer 4

Dig towards Windows Auto Maintenance. Or Windows Updates. There are options in Policy where it is indicated when it is possible to reboot and whether it is possible to reboot at all