Ubuntu Server 18.04 automatically rebooted but no logs per se

I have a hypervisor with Ubuntu Server 18.04, on which several virtual machines are installed, running on VirtualBox. At 23:45, the physical server automatically restarted. I tried to look in syslog and kern.log, but I did not find anything about the system restart Part of syslog at the moment of restart

Aug 29 23:38:27 hypervisor dhcpd: DHCPREQUEST for 12.34.56.78 from 08:00:27:75:09:8d via vboxnet0
Aug 29 23:38:27 hypervisor dhcpd: DHCPACK on 12.34.56.78 to 08:00:27:75:09:8d via vboxnet0
Aug 29 23:38:39 hypervisor named: resolver priming query complete
Aug 29 23:39:01 hypervisor CRON: (root) CMD (  [ -x /usr/lib/php/sessionclean ] && if [ ! -d /run/systemd/system ]; then /usr/lib/php/sessionclean; fi)
Aug 29 23:39:07 hypervisor dhcpd: DHCPREQUEST for 123.234.123.12 from 08:00:27:c3:0c:28 via vboxnet2
Aug 29 23:39:07 hypervisor dhcpd: DHCPACK on 123.234.123.12 to 08:00:27:c3:0c:28 via vboxnet2
Aug 29 23:39:17 hypervisor systemd: Starting Clean php session files...
Aug 29 23:39:18 hypervisor systemd: Started Clean php session files.
Aug 29 23:39:33 hypervisor named: resolver priming query complete
Aug 29 23:40:57 hypervisor dhcpd: DHCPREQUEST for 12.34.56.78 from 08:00:27:75:09:8d via vboxnet0
Aug 29 23:40:57 hypervisor dhcpd: DHCPACK on 12.34.56.78 to 08:00:27:75:09:8d via vboxnet0
Aug 29 23:41:37 hypervisor dhcpd: DHCPREQUEST for 123.234.123.12 from 08:00:27:c3:0c:28 via vboxnet2
Aug 29 23:41:37 hypervisor dhcpd: DHCPACK on 123.234.123.12 to 08:00:27:c3:0c:28 via vboxnet2
Aug 29 23:41:37 hypervisor named: resolver priming query complete
Aug 29 23:43:19 hypervisor named: message repeated 3 times: [ resolver priming query complete]
Aug 29 23:43:27 hypervisor dhcpd: DHCPREQUEST for 12.34.56.78 from 08:00:27:75:09:8d via vboxnet0
Aug 29 23:43:27 hypervisor dhcpd: DHCPACK on 12.34.56.78 to 08:00:27:75:09:8d via vboxnet0
Aug 29 23:43:33 hypervisor named: resolver priming query complete
Aug 29 23:44:07 hypervisor dhcpd: DHCPREQUEST for 123.234.123.12 from 08:00:27:c3:0c:28 via vboxnet2
Aug 29 23:44:07 hypervisor dhcpd: DHCPACK on 123.234.123.12 to 08:00:27:c3:0c:28 via vboxnet2
Aug 29 23:46:03 hypervisor systemd-modules-load: Inserted module 'coretemp'
Aug 29 23:46:03 hypervisor kernel: [    0.000000] microcode: microcode updated early to revision 0xf4, date = 2022-07-31
Aug 29 23:46:03 hypervisor kernel: [    0.000000] Linux version 4.15.0-213-generic (buildd@lcy02-amd64-079) (gcc version 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04)) #224-Ubuntu SMP Mon Jun 19 13:30:12 UTC 2023 (Ubuntu 4.15.0-213.224-generic 4.15.18)
Aug 29 23:46:03 hypervisor kernel: [    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-4.15.0-213-generic root=UUID=60a0068c-dde5-4583-8574-98c0a03bb007 ro net.ifnames=0
Aug 29 23:46:03 hypervisor systemd: Started Uncomplicated firewall.
Aug 29 23:46:03 hypervisor kernel: [    0.000000] KERNEL supported cpus:
Aug 29 23:46:03 hypervisor systemd: Starting Flush Journal to Persistent Storage...
Aug 29 23:46:03 hypervisor kernel: [    0.000000]   Intel GenuineIntel
Aug 29 23:46:03 hypervisor kernel: [    0.000000]   AMD AuthenticAMD
Aug 29 23:46:03 hypervisor kernel: [    0.000000]   Centaur CentaurHauls
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x001: 'x87 floating point registers'
Aug 29 23:46:03 hypervisor systemd: Started Set the console keyboard layout.
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x002: 'SSE registers'
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x004: 'AVX registers'
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x008: 'MPX bounds registers'
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x010: 'MPX CSR'
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x200: 'Protection Keys User registers'
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: xstate_offset:  576, xstate_sizes:  256
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: xstate_offset:  832, xstate_sizes:   64
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: xstate_offset:  896, xstate_sizes:   64
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: xstate_offset:  960, xstate_sizes:    8
Aug 29 23:46:03 hypervisor systemd: Started Load/Save Random Seed.
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: Enabled xstate features 0x21f, context size is 968 bytes, using 'compacted' format.

kern.log

Aug 26 17:24:07 hypervisor kernel: [4048716.348358] device vboxnet0 left promiscuous mode
Aug 26 17:24:07 hypervisor kernel: [4048716.348379] device vboxnet3 left promiscuous mode
Aug 26 17:24:07 hypervisor kernel: [4048716.418638] vboxnetflt: 8265681 out of 8655381 packets were not sent (directed to host)
Aug 26 17:24:07 hypervisor kernel: [4048716.470636] vboxnetflt: 388 out of 534983 packets were not sent (directed to host)
Aug 26 22:36:39 hypervisor kernel: [4067468.933959] VBoxNetFlt: attached to 'vboxnet0' / 0a:00:27:00:00:00
Aug 26 22:36:39 hypervisor kernel: [4067468.934478] VBoxNetFlt: attached to 'vboxnet3' / 0a:00:27:00:00:03
Aug 26 22:36:39 hypervisor kernel: [4067468.935191] VMMR0InitVM: eflags=246 fKernelFeatures=0x0 (SUPKERNELFEATURES_SMAP=0)
Aug 26 22:36:39 hypervisor kernel: [4067468.936917] device vboxnet0 entered promiscuous mode
Aug 26 22:36:39 hypervisor kernel: [4067468.936929] device vboxnet3 entered promiscuous mode
Aug 26 22:37:11 hypervisor kernel: [4067500.476672] VMMR0InitVM: eflags=246 fKernelFeatures=0x0 (SUPKERNELFEATURES_SMAP=0)
Aug 26 22:39:59 hypervisor kernel: [4067668.448094] device vboxnet0 left promiscuous mode
Aug 26 22:39:59 hypervisor kernel: [4067668.487732] vboxnetflt: 3543 out of 3649 packets were not sent (directed to host)
Aug 26 22:40:25 hypervisor kernel: [4067694.334590] VBoxNetFlt: attached to 'vboxnet0' / 0a:00:27:00:00:00
Aug 26 22:40:25 hypervisor kernel: [4067694.335071] device vboxnet0 entered promiscuous mode
Aug 26 22:42:31 hypervisor kernel: [4067820.931694] device vboxnet3 left promiscuous mode
Aug 26 22:42:31 hypervisor kernel: [4067821.036953] vboxnetflt: 0 out of 529 packets were not sent (directed to host)
Aug 29 23:46:03 hypervisor kernel: [    0.000000] microcode: microcode updated early to revision 0xf4, date = 2022-07-31
Aug 29 23:46:03 hypervisor kernel: [    0.000000] Linux version 4.15.0-213-generic (buildd@lcy02-amd64-079) (gcc version 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04)) #224-Ubuntu SMP Mon Jun 19 13:30:12 UTC 2023 (Ubuntu 4.15.0-213.224-generic 4.15.18)
Aug 29 23:46:03 hypervisor kernel: [    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-4.15.0-213-generic root=UUID=60a0068c-dde5-4583-8574-98c0a03bb007 ro net.ifnames=0
Aug 29 23:46:03 hypervisor kernel: [    0.000000] KERNEL supported cpus:
Aug 29 23:46:03 hypervisor kernel: [    0.000000]   Intel GenuineIntel
Aug 29 23:46:03 hypervisor kernel: [    0.000000]   AMD AuthenticAMD
Aug 29 23:46:03 hypervisor kernel: [    0.000000]   Centaur CentaurHauls
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x001: 'x87 floating point registers'
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x002: 'SSE registers'
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x004: 'AVX registers'
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x008: 'MPX bounds registers'
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x010: 'MPX CSR'
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x200: 'Protection Keys User registers'
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: xstate_offset:  576, xstate_sizes:  256
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: xstate_offset:  832, xstate_sizes:   64
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: xstate_offset:  896, xstate_sizes:   64
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: xstate_offset:  960, xstate_sizes:    8
Aug 29 23:46:03 hypervisor kernel: [    0.000000] x86/fpu: Enabled xstate features 0x21f, context size is 968 bytes, using 'compacted' format.
Aug 29 23:46:03 hypervisor kernel: [    0.000000] e820: BIOS-provided physical RAM map:
Aug 29 23:46:03 hypervisor kernel: [    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009dfff] usable
Aug 29 23:46:03 hypervisor kernel: [    0.000000] BIOS-e820: [mem 0x000000000009e000-0x000000000009efff] reserved
Aug 29 23:46:03 hypervisor kernel: [    0.000000] BIOS-e820: [mem 0x000000000009f000-0x000000000009ffff] usable

last reboot says (after an unscheduled restart, I rebooted myself once):

reboot   system boot  4.15.0-213-gener Fri Aug 30 00:03   still running
reboot   system boot  4.15.0-213-gener Thu Aug 29 23:45 - 00:03  (00:17)

wtmp begins Thu Aug  1 13:43:08 2024

cat /proc/sys/kernel/panic is 0 The physical server has been running for about 40 days without rebooting. How can I find out the reason for the reboot and prevent this from happening in the future?