I can't pinpoint the cause of the freeze exactly, but it shows as follows in syslog:
At midnight, logrotate was started but never successfully concluded.
At the time, I was also recording a stream via smb to the hdd. (Not a very demanding activity)
When I tried to access the SBC via network share or ssh this morning, neither would show as available. The SBC was running but "dead".
When I rebooted it by re-plugging power, instead of showing ~9:00 o'clock, the SBCs internal clock showed 02:17.
Here are the last messages of the syslog before the SBC locked up:
I should note that the SBC has been running 24/7 stable for weeks.
There have been more hardware demanding tasks running without issue.