Let me first correct a statement I made in the initial report. I stated that it looked like the problems did not occur under openSUSE Leap 15.1. More careful analysis of the log messages showed that the problems did occur under 15.1 as well, though there were far, far less log messages, which is why I overlooked them initially. I started working on a hunch that this may be a firmware issue. So I first upgraded the firmware of the hard drives to version A925. That did not solve the problems. Next I upgraded the firmware of the SAS controller card to version 16.00.10.00 (IT version -- there are two versions of the firmware, one with simple RAID support, and another that simply exports the disks without implementing any RAID solutions -- the IT version is the latter). This seems to have solved the problems. There have been no more error messages since the firmware upgrade.