Comment # 5 on bug 1158241 from Ulrich Windl

(In reply to Franck Bui from comment #3)
> Can you find something in /var/lib/systemd/coredump ?

Yes, I have two of them, and the first one seems to match:
total 628
-rw-r----- 1 root root 336677 Dec  2 12:49
core.systemd-journal.0.e2ee8dc29c0f4579a2f8ce0627b51f03.1726.1575287282000000.lz4
-rw-r----- 1 root root 300826 Dec  4 08:48
core.systemd-journal.0.e2ee8dc29c0f4579a2f8ce0627b51f03.21398.1575445623000000.lz4

And for the second core-dump the events looked like this:
(The system was up for quite a while then)
Dec 04 08:48:46 pc systemd-journald[27776]: Journal started
Dec 04 08:48:46 pc systemd-journald[27776]: System journal
(/var/log/journal/89c660865c00403a9bacef32b6828556) is 840.0M, max 819.2M, 0B
free.
...
Dec 04 08:48:46 pc systemd-coredump[27757]: Process 21398 (systemd-journal) of
user 0 dumped core.
Dec 04 08:48:46 pc systemd-coredump[27757]: Coredump diverted to
/var/lib/systemd/coredump/core.systemd-journal.0.e2ee8dc29c0f4579a2f8ce0627b51f03.21398.1575445623000000.l>
Dec 04 08:48:46 pc systemd-coredump[27757]: Stack trace of thread 21398:
Dec 04 08:48:46 pc systemd-coredump[27757]: #0  0x00007f2260b49750 n/a
(libsystemd-shared-234.so)
Dec 04 08:48:46 pc systemd-coredump[27757]: #1  0x00007f2260b4b619
journal_file_append_entry (libsystemd-shared-234.so)
Dec 04 08:48:46 pc systemd-coredump[27757]: #2  0x0000560b8230df8d n/a
(systemd-journald)
Dec 04 08:48:46 pc systemd-coredump[27757]: #3  0x0000560b8231005f n/a
(systemd-journald)
Dec 04 08:48:46 pc systemd-coredump[27757]: #4  0x0000560b82307ed2 n/a
(systemd-journald)
Dec 04 08:48:46 pc systemd-coredump[27757]: #5  0x00007f2260b1eae8 n/a
(libsystemd-shared-234.so)
Dec 04 08:48:46 pc systemd-coredump[27757]: #6  0x00007f2260b20222
sd_event_dispatch (libsystemd-shared-234.so)
Dec 04 08:48:46 pc kernel: systemd-coredum: 4 output lines suppressed due to
ratelimiting
Dec 04 08:48:46 pc packagekitd[27495]:
DownloadProgressReportReceiver::start():http://download.opensuse.org/update/leap/15.1/oss/
--libarchive13;3.3.2-lp151.5.3.1;x86_64;r>
Dec 04 08:47:02 pc systemd[1]: systemd-journald.service: Watchdog timeout
(limit 3min)!
Dec 04 08:47:02 pc systemd[1]: systemd-journald.service: Killing process 21398
(systemd-journal) with signal SIGABRT.
Dec 04 08:48:46 pc systemd[1]: Starting Flush Journal to Persistent Storage...
Dec 04 08:48:46 pc systemd[1]: Started Flush Journal to Persistent Storage.
...