jails system powered off on its own, twice in about 2 weeks

I had the same issue appearing a few months ago, without any significant system change.

I swapped the (rather old) PSU to a (similarly old but) stronger one, and the issue went away. Strangely, the power-offs did usually not occur at full load, and the respective PSU is now driving the desktop without problems.

Lesson learned: these PSU do degrade over time (at least when running with serious load) and what was calculated to work and did once work, may suddenly start to produce random failures.