the affected system is still up and in the not-yet rebooted state. I can grab
journalctl from around that timeframe:

May 26 22:30:01 dpg-ppb-smu-tux01 cron[11597]: pam_unix(crond:session): session
opened for user root by (uid=0)
May 26 22:30:01 dpg-ppb-smu-tux01 systemd[1]: Created slice User Slice of root.
-- Subject: Unit user-0.slice has finished start-up
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit user-0.slice has finished starting up.
-- 
-- The start-up result is done.
May 26 22:30:01 dpg-ppb-smu-tux01 systemd[1]: Starting User Manager for UID
0...
-- Subject: Unit user@0.service has begun start-up
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit user@0.service has begun starting up.
May 26 22:30:01 dpg-ppb-smu-tux01 systemd[11598]:
pam_unix(systemd-user:session): session opened for user root by (uid=0)
May 26 22:30:01 dpg-ppb-smu-tux01 systemd[1]: Started Session 920 of user root.
-- Subject: Unit session-920.scope has finished start-up
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit session-920.scope has finished starting up.
-- 
-- The start-up result is done.
May 26 22:30:01 dpg-ppb-smu-tux01 systemd[11598]: Reached target Timers.
-- Subject: Unit UNIT has finished start-up
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit user@0.service has begun shutting down.
May 26 22:30:01 dpg-ppb-smu-tux01 systemd[11598]: Reached target Shutdown.
-- Subject: Unit UNIT has finished start-up
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit UNIT has finished starting up.
-- 
-- The start-up result is done.
May 26 22:30:01 dpg-ppb-smu-tux01 systemd[11598]: Stopped target Default.
-- Subject: Unit UNIT has finished shutting down
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit UNIT has finished shutting down.
May 26 22:30:01 dpg-ppb-smu-tux01 systemd[11598]: Stopped target Basic System.
-- Subject: Unit UNIT has finished shutting down
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit UNIT has finished shutting down.
May 26 22:30:01 dpg-ppb-smu-tux01 systemd[11598]: Stopped target Timers.
-- Subject: Unit UNIT has finished shutting down
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit UNIT has finished shutting down.
May 26 22:30:01 dpg-ppb-smu-tux01 systemd[11598]: Stopped target Paths.
-- Subject: Unit UNIT has finished shutting down
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit UNIT has finished shutting down.
May 26 22:30:01 dpg-ppb-smu-tux01 systemd[11598]: Stopped target Sockets.
-- Subject: Unit UNIT has finished shutting down
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit UNIT has finished shutting down.
May 26 22:30:01 dpg-ppb-smu-tux01 systemd[11598]: Starting Exit the Session...
-- Subject: Unit UNIT has begun start-up
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit UNIT has begun starting up.
May 26 22:30:01 dpg-ppb-smu-tux01 systemd[11598]: Received SIGRTMIN+24 from PID
11643 (kill).
May 26 22:30:01 dpg-ppb-smu-tux01 systemd[11600]:
pam_unix(systemd-user:session): session closed for user root
May 26 22:30:01 dpg-ppb-smu-tux01 systemd[1]: Stopped User Manager for UID 0.
-- Subject: Unit user@0.service has finished shutting down
....

Any more info needed?