18.04 – Possible cause of an unexpected shutdown?

My computer was recently shut down unexpectedly for no apparent reason. the latest The command indicates that the system has crashed, but I found no reason for this in the system logs:

last exit:

reboot the system 4.15.0-46-generi Thu 28 Mar 10:02 - 10:22 (00:20)
User name tty2 tty2 Thu 28 Mar 08:39 - crash (01:22)
reboot the system 4.15.0-46-generi Thu 28 Mar 08:39 - 10:22 (01:42)

syslog:

March 28 09:53:11 U-C-001 gdm-session-worker[1589]: modprobe: ERROR: Can not insert 'nvidia': key required not available
March 28th 09:53:11 Core U-C-001: [ 4453.744318] Unsigned PKCS # 7 signature with trust key
March 28 9:53:14 U-C-001 software gnome[2757]: no application for ubuntu-dock@ubuntu.com modified
March 28 9:53:14 U-C-001 software gnome[2757]: no application for modified ubuntu-appindicators@ubuntu.com
March 28 09:53:14 U-C-001 gnome-shell[1675]: [AppIndicatorSupport-DEBUG] Registering StatusNotifierItem: 1.75 / org / ayatana / NotificationItem / software_update_available
March 28 09:53:16 U-C-001 firefox_firefox.desktop[6462]: / snap / firefox / 198 / bin / desktop-launch: line 174: /home/username/snap/firefox/common/.config/user-dirs.dirs: no such file or directory
March 28 09:53:16 U-C-001 firefox_firefox.desktop[6462]: cp: Can not create the normal file: /home/username/snap/firefox/common/.config /: not a directory
March 28 09:53:16 U-C-001 firefox_firefox.desktop[6462]: / snap / firefox / 198 / bin / desktop-launch: line 177: /home/username/snap/firefox/common/.config/user-dirs.dirs.md5sum: no such file or directory
March 28 09:53:16 U-C-001 firefox_firefox.desktop[6462]: / snap / firefox / 198 / bin / desktop-launch: line 177: /home/username/snap/firefox/common/.config/user-dirs.locale.md5sum: no such file or directory
March 28 09:53:16 U-C-001 firefox_firefox.desktop[6462]: Gtk-Message: Failed to load module "canberra-gtk-module"
March 28 09:53:16 Core U-C-001: [ 4458.808586] audit: type = 1400 audit (1553763196.420: 60): apparmor = "REFUSAL" operation = "open" profile = "snap.firefox.firefox" name = "/ proc / 6462 / net / arp" pid = 6462 comm = 4C696E6B204D6F6E6976F72 requested_mask = "r" denied_mask = "r" fsuid = 1000 ouid = 0
March 28 09:53:16 U-C-001 firefox_firefox.desktop[6462]: Gtk-Message: Failed to load module "canberra-gtk-module"
March 28 09:53:16 U-C-001 firefox_firefox.desktop[6462]: (firefox: 6462): Gtk-CRITICAL **: gtk_widget_get_style_context: the assertion GTK_IS_WIDGET (widget) & # 39; has failed
March 28 09:53:16 U-C-001 firefox_firefox.desktop[6462]: (firefox: 6462): Gtk-CRITICAL **: gtk_style_context_get_state: assertion & # 39; GTK_IS_STYLE_CONTEXT (context) & # 39; has failed
March 28 09:53:16 U-C-001 firefox_firefox.desktop[6462]: (firefox: 6462): Gtk-CRITICAL **: gtk_style_context_set_state: assertion & # 39; GTK_IS_STYLE_CONTEXT (context) & # 39; has failed
March 28 09:53:16 U-C-001 firefox_firefox.desktop[6462]: (firefox: 6462): Gtk-CRITICAL **: gtk_render_background: assertion & # 39; GTK_IS_STYLE_CONTEXT (context) & # 39; has failed
March 28 09:53:16 U-C-001 firefox_firefox.desktop[6462]: (firefox: 6462): Gtk-CRITICAL **: gtk_render_frame: assertion & # 39; GTK_IS_STYLE_CONTEXT (context) & # 39; has failed.
March 28 09:53:16 U-C-001 firefox_firefox.desktop[6462]: Gtk-Message: Failed to load module "canberra-gtk-module"
March 28 09:53:16 U-C-001 firefox_firefox.desktop[6462]: Gtk-Message: Failed to load module "canberra-gtk-module"
March 28 09:53:17 U-C-001 gnome-shell[1675]: JS ERROR: Exception in callback for signal: activate: Error: Error while calling IBus.set_global_engine_async: Expected function for recall of callback argument has been undefined # 012setEngine @ resource : ///org/gnome/shell/misc/ibusManager.js: 207: 9 # 012wrapper @ resource: ///org/gnome/gjs/modules/_legacy.js: 82: 22 # 012activateInputSource @ resource: /// org / gnome / shell / ui / status / keyboard.js: 490: 13 # 012wrapper @ resource: ///org/gnome/gjs/modules/_legacy.js: 82: 22 # 012_emit @ resource: /// org / gnome / gjs / modules / signals.js: 128: 27 # 012activate @ resource: ///org/gnome/shell/ui/status/keyboard.js: 65: 9 # 012wrapper @ resource: /// org / gnome / #### modules / _legacy.js: 82: 22 # 012reload @ resource: /// org / gnome / shell / ui / status / keyboard.js: 369: 9 # 012wrapper @ resource: /// org / gnome / gjs / modu / _legacy.js: 82: 22 # 012_ibusSetContentType @ resource: /// org / gnome / shell / ui / status / keyboard.js: 691: 9 # 012wrapper @ resour ce: /// org / gnome / gjs / modules /_legacy.js: 82: 22 # 012_emit @ resource: ///org/gnome/gjs/modules/signals.js:128:27 # 012_setContentType @ resource: // /org/gnome/shell/misc/ibusManager.js 183: # 9 @ 012wrapper resource: ///org/gnome/gjs/modules/_legacy.js: 82: 22
March 28 09:53:17 U-C-001 firefox_firefox.desktop[6462]: Gtk-Message: Failed to load module "canberra-gtk-module"
March 28 09:53:18 U-C-001 firefox_firefox.desktop[6462]: message repeated 3 times: [ Gtk-Message: Failed to load module "canberra-gtk-module"]
March 28 09:53:29 U-C-001 gdm-session-worker[1589]: modprobe: ERROR: Can not insert 'nvidia': key required not available
March 28 09:53:29 Core U-C-001: [ 4471.952558] Unsigned PKCS # 7 signature with trust key
March 28 09:53:32 U-C-001 firefox_firefox.desktop[6462]: Gtk-Message: Failed to load module "canberra-gtk-module"
March 28 09:53:32 U-C-001 firefox_firefox.desktop[6462]: Gtk-Message: Failed to load module "canberra-gtk-module"
March 28 09:54:45 U-C-001 org.gnome.Shell.desktop[1675]: Window manager warning: last_user_time (4548485) is greater than the comparison time stamp (4548484). This probably represents a buggy client that sends inaccurate timestamps in messages such as _NET_ACTIVE_WINDOW. Trying to work around ...
March 28 09:54:45 U-C-001 org.gnome.Shell.desktop[1675]: Window Manager Warning: 0x1c00010 ([firefox website name]) seems to be one of the offending windows with a timestamp of 4548485. Bypassing ...
March 28 09:54:46 U-C-001 org.gnome.Shell.desktop[1675]: Window manager warning: last_user_time (4548515) is greater than the comparison time stamp (4548514). This probably represents a buggy client that sends inaccurate timestamps in messages such as _NET_ACTIVE_WINDOW. Trying to work around ...
March 28 09:54:46 U-C-001 org.gnome.Shell.desktop[1675]: Window Manager Warning: 0x1c00010 ([firefox website name]) seems to be one of the offending windows with a timestamp of 4548515. Bypassing ...
March 28 09:54:49 U-C-001 org.gnome.Shell.desktop[1675]: Window manager warning: last_user_time (4551614) is greater than the comparison time stamp (4551613). This probably represents a buggy client that sends inaccurate timestamps in messages such as _NET_ACTIVE_WINDOW. Trying to work around ...
March 28 09:54:49 U-C-001 org.gnome.Shell.desktop[1675]: Window Manager Warning: 0x1c00010 ([firefox website name]) seems to be one of the offending windows with a timestamp of 4551614. Bypassing ...
March 28 09:54:52 U-C-001 org.gnome.Shell.desktop[1675]: Window manager warning: last_user_time (4555028) is greater than the comparison time stamp (4555027). This probably represents a buggy client that sends inaccurate timestamps in messages such as _NET_ACTIVE_WINDOW. Trying to work around ...
March 28 09:54:52 U-C-001 org.gnome.Shell.desktop[1675]: Window Manager Warning: 0x1c00010 ([firefox website name]) seems to be one of the incriminated windows with a time stamp of 4555028. When working ...
March 28 09:59:39 U-C-001 core: [ 4841.893118] [UFW BLOCK]    IN = wlp3s0 OUT = MAC =[MAC] SRC =[router internal IP] DST = 224.0.0.1 LEN = 32 TOS = 0x00 PREC = 0xC0 TTL = 1 ID = 15777 PROTO = 2
March 28 10:02:33 U-C-001 systemd[1]: Started udev Coldplug all devices.
March 28 10:02:33 Core U-C-001: [    0.000000] microcode: firmware updated early to revision 0x8e, date = 2018-03-24
March 28 10:02:33 Core U-C-001: [    0.000000] Linux version 4.15.0-46-generic (buildd @ lgw01-amd64-038) (gcc version 7.3.0 (Ubuntu 7.3.0-16ubuntu3)) # 49-Ubuntu SMP Wed Feb 6 09:33:07 UTC 2019 (Ubuntu 4.15.0-46.49-generic 4.15.18)

Although there are several errors in the syslog, they all occurred several minutes before the shutdown and seem not to be related to it. Is there a way to further investigate the issue? Do you have any idea of ​​what might have caused the closure? I am under Ubuntu on 18.04.02.