After putting the disks of the 7-stable system which exhibited stability problems into a completely different system (it is a rented root-server, not our own hardware), the system now survived more than a day (and still no trace of problems) with the UFS setup. Previously it would crash after some minutes.
The ZFS setup with the changed hardware had a problem during the night before (like always after all my ZFS related changes on this machine), but on this machine I changed all locks in ZFS from shared locks to exclusive locks (this extended the uptime from 4 – 6 hours to “until I rebooted the morning after because of hanging processes”), so this may be because of this. I do not know yet if we will test the ZFS setup with the pure 7‑stable source we use now or not (the goal was to get back a stable system, instead of playing around with unrelated stuff).
It looks like some kind of hardware problem was uncovered by updating from 7.1 to 7.2 (and 7‑stable subsequently). This new machine has a completely different chipset, a new CPU and RAM and PSU and … so I do not really know what caused this (but the fact that the previous system did not recognize the CPU after replacing it with a bigger one and the observation that only shared locks with a specific usage pattern where affected lets me point towards missing microcode updates…).