It seems the machine was target of some distributed attack, and as a
consequence systemd-journal seems to have crashed. I found these entries:
Dec 02 12:49:57 pc systemd-coredump[21380]: Process 1726 (systemd-journal) of
user 0 dumped core.
Dec 02 12:49:57 pc systemd-coredump[21380]: Coredump diverted to
/var/lib/systemd/coredump/core.systemd-journal.0.e2ee8dc29c0f4579a2f8ce0627b51f03.1726.15752872820...
Dec 02 12:49:57 pc systemd-coredump[21380]: Stack trace of thread 1726:
Dec 02 12:49:57 pc systemd-coredump[21380]: #0  0x00007f4cbaab9230
journal_file_find_data_object_with_hash (libsystemd-shared-234.so)
Dec 02 12:49:57 pc systemd-coredump[21380]: #1  0x00007f4cbaab94bd n/a
(libsystemd-shared-234.so)
Dec 02 12:49:57 pc systemd-coredump[21380]: #2  0x00007f4cbaabb619
journal_file_append_entry (libsystemd-shared-234.so)
Dec 02 12:49:57 pc systemd-coredump[21380]: #3  0x0000556185680f8d n/a
(systemd-journald)
Dec 02 12:49:57 pc systemd-coredump[21380]: #4  0x000055618568305f n/a
(systemd-journald)
Dec 02 12:49:57 pc systemd-coredump[21380]: #5  0x000055618567aed2 n/a
(systemd-journald)
Dec 02 12:49:57 pc systemd-coredump[21380]: #6  0x00007f4cbaa8eae8 n/a
(libsystemd-shared-234.so)
Dec 02 12:49:57 pc kernel: systemd-coredum: 5 output lines suppressed due to
ratelimiting
Dec 02 12:49:57 pc kernel: SFW2-INext-DROP-DEFLT IN=eth0 OUT=
MAC=00:24:1d:d7:0b:17:bc:ea:fa:a4:79:00:08:00 SRC=92.53.104.212 DST=<IP> LEN=40
TOS=0x00 P...
Dec 02 12:49:57 pc systemd-journald[21398]: Journal started
Dec 02 12:49:57 pc systemd-journald[21398]: System journal
(/var/log/journal/89c660865c00403a9bacef32b6828556) is 832.0M, max 819.2M, 0B
free.
Dec 02 12:48:02 pc systemd[1]: systemd-journald.service: Watchdog timeout
(limit 3min)!
Dec 02 12:48:02 pc systemd[1]: systemd-journald.service: Killing process 1726
(systemd-journal) with signal SIGABRT.