I found this very useful answer at first I thought it was because of unattended upgrades, then followed recommendations in this answer, but the system still shuts down.
Below are partial outputs of the mentioned commands, I don't know what else to do before actually changing hardware.
Thank you very much everyone!
This is the output of: last -x | head | tac
runlevel (to lvl 5) 4.15.0-128-gener Tue Jan 5 22:52 - 22:59 (00:06)
reboot system boot 4.15.0-128-gener Tue Jan 5 22:57 still running
runlevel (to lvl 5) 4.15.0-128-gener Tue Jan 5 22:59 - 00:50 (01:51)
reboot system boot 4.15.0-128-gener Wed Jan 6 00:49 still running
runlevel (to lvl 5) 4.15.0-128-gener Wed Jan 6 00:50 still running
user1 pts/1 192.168.1.20 Wed Jan 6 03:44 still logged in
user1 pts/0 :pts/1:S.0 Wed Jan 6 03:59 - 03:59 (00:00)
user1 pts/0 :pts/1:S.0 Wed Jan 6 03:59 - 04:09 (00:09)
user1 pts/0 :pts/1:S.0 Wed Jan 6 04:10 - 04:11 (00:00)
user1 pts/0 :pts/1:S.0 Wed Jan 6 04:43 - 05:14 (00:31)
And this is part of the output of
grep -iv ': starting\|kernel: .*: Power Button\|watching system buttons\|Stopped Cleaning Up\|Started Crash recovery kernel' /var/log/messages /var/log/syslog /var/log/apcupsd* | grep -iw 'recover[a-z]*\|power[a-z]*\|shut[a-z ]*down\|rsyslogd\|ups'
grep: /var/log/messages: No such file or directory
/var/log/syslog:Jan 5 18:07:56 medesvr1 kernel: [ 0.049508] ACPI: Power Resource [USBC] (on)
/var/log/syslog:Jan 5 18:07:56 medesvr1 kernel: [ 0.053135] ACPI: Power Resource [PLPE] (on)
/var/log/syslog:Jan 5 18:07:56 medesvr1 kernel: [ 0.053994] ACPI: Power Resource [PLPE] (on)
/var/log/syslog:Jan 5 18:07:56 medesvr1 kernel: [ 0.068250] ACPI: Power Resource [CLK0] (on)
/var/log/syslog:Jan 5 18:07:56 medesvr1 kernel: [ 0.068356] ACPI: Power Resource [CLK1] (on)
/var/log/syslog:Jan 5 18:07:56 medesvr1 kernel: [ 0.235790] pci 0000:01:00.1: D0 power state depends on 0000:01:00.0
/var/log/syslog:Jan 5 18:07:56 medesvr1 systemd[1]: Started Update UTMP about System Boot/Shutdown.
/var/log/syslog:Jan 5 18:07:56 medesvr1 apparmor[596]: Skipping profile in /etc/apparmor.d/disable: usr.sbin.rsyslogd
/var/log/syslog:Jan 5 18:07:56 medesvr1 dbus-daemon[900]: dbus[900]: Unknown group "power" in message bus configuration file
/var/log/syslog:Jan 5 18:07:56 medesvr1 kernel: [ 4.852026] raid6: using ssse3x2 recovery algorithm
/var/log/syslog:Jan 5 18:07:56 medesvr1 kernel: [ 5.518198] EXT4-fs (sdb2): INFO: recovery required on readonly filesystem
/var/log/syslog:Jan 5 18:07:56 medesvr1 kernel: [ 5.518208] EXT4-fs (sdb2): write access will be enabled during recovery
/var/log/syslog:Jan 5 18:07:56 medesvr1 kernel: [ 17.492117] EXT4-fs (sdb2): recovery complete
/var/log/syslog:Jan 5 18:07:56 medesvr1 rsyslogd: imuxsock: Acquired UNIX socket '/run/systemd/journal/syslog' (fd 3) from systemd. [v8.32.0]
/var/log/syslog:Jan 5 18:07:56 medesvr1 rsyslogd: rsyslogd's groupid changed to 106
/var/log/syslog:Jan 5 18:07:56 medesvr1 rsyslogd: rsyslogd's userid changed to 102
/var/log/syslog:Jan 5 18:07:56 medesvr1 rsyslogd: [origin software="rsyslogd" swVersion="8.32.0" x-pid="935" x-info="http://www.rsyslog.com"] start
/var/log/syslog:Jan 5 18:07:57 medesvr1 systemd[1]: Started Unattended Upgrades Shutdown.
/var/log/syslog:Jan 5 18:08:36 medesvr1 set-cpufreq[897]: Setting powersave scheduler for all CPUs
/var/log/syslog:Jan 5 18:08:41 medesvr1 systemd[1]: Started LXD - container startup/shutdown.
/var/log/syslog:Jan 5 22:52:16 medesvr1 systemd[1]: Started Update UTMP about System Boot/Shutdown.
/var/log/syslog:Jan 5 22:52:16 medesvr1 apparmor[599]: Skipping profile in /etc/apparmor.d/disable: usr.sbin.rsyslogd
/var/log/syslog:Jan 5 22:52:16 medesvr1 kernel: [ 0.049486] ACPI: Power Resource [USBC] (on)
/var/log/syslog:Jan 5 22:52:16 medesvr1 kernel: [ 0.053108] ACPI: Power Resource [PLPE] (on)
/var/log/syslog:Jan 5 22:52:16 medesvr1 kernel: [ 0.053967] ACPI: Power Resource [PLPE] (on)
/var/log/syslog:Jan 5 22:52:16 medesvr1 kernel: [ 0.068215] ACPI: Power Resource [CLK0] (on)
/var/log/syslog:Jan 5 22:52:16 medesvr1 kernel: [ 0.068321] ACPI: Power Resource [CLK1] (on)
/var/log/syslog:Jan 5 22:52:16 medesvr1 kernel: [ 0.235872] pci 0000:01:00.1: D0 power state depends on 0000:01:00.0
/var/log/syslog:Jan 5 22:52:16 medesvr1 kernel: [ 4.920040] raid6: using ssse3x2 recovery algorithm
/var/log/syslog:Jan 5 22:52:16 medesvr1 kernel: [ 5.576970] EXT4-fs (sdb2): INFO: recovery required on readonly filesystem
/var/log/syslog:Jan 5 22:52:16 medesvr1 kernel: [ 5.576980] EXT4-fs (sdb2): write access will be enabled during recovery
/var/log/syslog:Jan 5 22:52:16 medesvr1 kernel: [ 44.073736] EXT4-fs (sdb2): recovery complete
/var/log/syslog:Jan 5 22:52:16 medesvr1 dbus-daemon[905]: dbus[905]: Unknown group "power" in message bus configuration file
/var/log/syslog:Jan 5 22:52:16 medesvr1 systemd[1]: Started Unattended Upgrades Shutdown.
/var/log/syslog:Jan 5 22:52:16 medesvr1 set-cpufreq[985]: Setting powersave scheduler for all CPUs
/var/log/syslog:Jan 5 22:52:16 medesvr1 rsyslogd: imuxsock: Acquired UNIX socket '/run/systemd/journal/syslog' (fd 3) from systemd. [v8.32.0]
/var/log/syslog:Jan 5 22:52:16 medesvr1 rsyslogd: rsyslogd's groupid changed to 106
/var/log/syslog:Jan 5 22:52:16 medesvr1 rsyslogd: rsyslogd's userid changed to 102
/var/log/syslog:Jan 5 22:52:16 medesvr1 rsyslogd: [origin software="rsyslogd" swVersion="8.32.0" x-pid="890" x-info="http://www.rsyslog.com"] start
/var/log/syslog:Jan 5 22:52:29 medesvr1 systemd[1]: Started LXD - container startup/shutdown.
/var/log/syslog:Jan 5 22:58:22 medesvr1 systemd[1]: Started Update UTMP about System Boot/Shutdown.
/var/log/syslog:Jan 5 22:58:22 medesvr1 apparmor[600]: Skipping profile in /etc/apparmor.d/disable: usr.sbin.rsyslogd
/var/log/syslog:Jan 5 22:58:22 medesvr1 dbus-daemon[918]: dbus[918]: Unknown group "power" in message bus configuration file
/var/log/syslog:Jan 5 22:58:22 medesvr1 systemd[1]: Started Unattended Upgrades Shutdown.
/var/log/syslog:Jan 5 22:58:22 medesvr1 rsyslogd: imuxsock: Acquired UNIX socket '/run/systemd/journal/syslog' (fd 3) from systemd. [v8.32.0]
/var/log/syslog:Jan 5 22:58:22 medesvr1 rsyslogd: rsyslogd's groupid changed to 106
/var/log/syslog:Jan 5 22:58:22 medesvr1 rsyslogd: rsyslogd's userid changed to 102
/var/log/syslog:Jan 5 22:58:22 medesvr1 rsyslogd: [origin software="rsyslogd" swVersion="8.32.0" x-pid="967" x-info="http://www.rsyslog.com"] start
/var/log/syslog:Jan 5 22:58:22 medesvr1 kernel: [ 0.049489] ACPI: Power Resource [USBC] (on)
/var/log/syslog:Jan 5 22:58:22 medesvr1 kernel: [ 0.053111] ACPI: Power Resource [PLPE] (on)
/var/log/syslog:Jan 5 22:58:22 medesvr1 kernel: [ 0.053967] ACPI: Power Resource [PLPE] (on)
/var/log/syslog:Jan 5 22:58:22 medesvr1 kernel: [ 0.068203] ACPI: Power Resource [CLK0] (on)
/var/log/syslog:Jan 5 22:58:22 medesvr1 kernel: [ 0.068309] ACPI: Power Resource [CLK1] (on)
/var/log/syslog:Jan 5 22:58:22 medesvr1 kernel: [ 0.235732] pci 0000:01:00.1: D0 power state depends on 0000:01:00.0
/var/log/syslog:Jan 5 22:58:22 medesvr1 kernel: [ 4.540036] raid6: using ssse3x2 recovery algorithm
/var/log/syslog:Jan 5 22:58:22 medesvr1 kernel: [ 5.203817] EXT4-fs (sdb2): INFO: recovery required on readonly filesystem
/var/log/syslog:Jan 5 22:58:22 medesvr1 kernel: [ 5.203826] EXT4-fs (sdb2): write access will be enabled during recovery
/var/log/syslog:Jan 5 22:58:22 medesvr1 kernel: [ 36.208830] EXT4-fs (sdb2): recovery complete
/var/log/syslog:Jan 5 22:58:57 medesvr1 set-cpufreq[965]: Setting powersave scheduler for all CPUs
/var/log/syslog:Jan 5 22:59:03 medesvr1 systemd[1]: Started LXD - container startup/shutdown.
/var/log/syslog:Jan 6 00:49:58 medesvr1 systemd[1]: Started Update UTMP about System Boot/Shutdown.
/var/log/syslog:Jan 6 00:49:58 medesvr1 apparmor[593]: Skipping profile in /etc/apparmor.d/disable: usr.sbin.rsyslogd
/var/log/syslog:Jan 6 00:49:58 medesvr1 dbus-daemon[891]: dbus[891]: Unknown group "power" in message bus configuration file
/var/log/syslog:Jan 6 00:49:58 medesvr1 systemd[1]: Started Unattended Upgrades Shutdown.
/var/log/syslog:Jan 6 00:49:58 medesvr1 kernel: [ 0.049501] ACPI: Power Resource [USBC] (on)
/var/log/syslog:Jan 6 00:49:58 medesvr1 kernel: [ 0.053123] ACPI: Power Resource [PLPE] (on)
/var/log/syslog:Jan 6 00:49:58 medesvr1 kernel: [ 0.053983] ACPI: Power Resource [PLPE] (on)
/var/log/syslog:Jan 6 00:49:58 medesvr1 kernel: [ 0.068240] ACPI: Power Resource [CLK0] (on)
/var/log/syslog:Jan 6 00:49:58 medesvr1 kernel: [ 0.068346] ACPI: Power Resource [CLK1] (on)
/var/log/syslog:Jan 6 00:49:58 medesvr1 kernel: [ 0.235940] pci 0000:01:00.1: D0 power state depends on 0000:01:00.0
/var/log/syslog:Jan 6 00:49:58 medesvr1 kernel: [ 4.560044] raid6: using ssse3x2 recovery algorithm
/var/log/syslog:Jan 6 00:49:58 medesvr1 kernel: [ 5.188958] EXT4-fs (sdb2): INFO: recovery required on readonly filesystem
/var/log/syslog:Jan 6 00:49:58 medesvr1 kernel: [ 5.188969] EXT4-fs (sdb2): write access will be enabled during recovery
/var/log/syslog:Jan 6 00:49:58 medesvr1 kernel: [ 32.240760] EXT4-fs (sdb2): recovery complete
/var/log/syslog:Jan 6 00:49:58 medesvr1 rsyslogd: imuxsock: Acquired UNIX socket '/run/systemd/journal/syslog' (fd 3) from systemd. [v8.32.0]
/var/log/syslog:Jan 6 00:49:58 medesvr1 rsyslogd: rsyslogd's groupid changed to 106
/var/log/syslog:Jan 6 00:49:58 medesvr1 rsyslogd: rsyslogd's userid changed to 102
/var/log/syslog:Jan 6 00:49:58 medesvr1 rsyslogd: [origin software="rsyslogd" swVersion="8.32.0" x-pid="928" x-info="http://www.rsyslog.com"] start
grep: /var/log/apcupsd*: No such file or directory
/var/log/syslog:Jan 6 00:50:05 medesvr1 set-cpufreq[885]: Setting powersave scheduler for all CPUs
/var/log/syslog:Jan 6 00:50:28 medesvr1 systemd[1]: Started LXD - container startup/shutdown.
/etc/apt/apt.conf.d/50unattended-upgrades, including reboot-if-needed (DO NOT REBOOT is the default setting). There seems no need to disable UU, which provides a valuable service to your system. One imagines you would remember (or, better yet, record) if you had changed the UU settings. – user535733 Jan 06 '21 at 07:33