My theory is that this is some kind of race condition. I initially cloned the machine from another one where this happened.
Then I discovered that the problem also happens (randomly) in a brand new machine if I copy the journal from the "bad" machine. However, there is nothing special about the journal (or at least "journalctl --verify" says it's ok), except maybe that it's several megabytes long. Could it be that systemd spends some time processing the journal at boot time and this is what triggers the race condition? Thanks.
