Woosah

Just documenting the solution here in case someone else needs it:
The base problem here was the same as with hammersandwhich findings. I have an AMD Ryzen 7 Pro 4750G CPU, which ran completely flawless until the kernel update which was introduced in the unraid update to 7.1. It was a bit obscured by a not-so-great SATA expansion card which I switched to a LSI 9300i one recently. Since then, the error logs and messages changed from something like the above to something like this:
Oct 13 13:54:37 Server kernel: mce: [Hardware Error]: Machine check events logged Oct 13 13:54:37 Server kernel: [Hardware Error]: Corrected error, no action required. Oct 13 13:54:37 Server kernel: [Hardware Error]: CPU:2 (17:60:1) MC1_STATUS[Over|CE|MiscV|AddrV|-|-|SyndV|-|-|-]: 0xdc20000000010859 Oct 13 13:54:37 Server kernel: [Hardware Error]: Error Addr: 0x000000020f818c40 Oct 13 13:54:37 Server kernel: [Hardware Error]: IPID: 0x000100b000000000, Syndrome: 0x000000005a020300 Oct 13 13:54:37 Server kernel: [Hardware Error]: Instruction Fetch Unit Ext. Error Code: 1 Oct 13 13:54:37 Server kernel: [Hardware Error]: cache level: L1, mem/io: IO, mem-tx: IRD, part-proc: SRC (no timeout)This gave me the same hint as hammersandwhich, that the kernel update did something, and it seemed to have something to do with the L1 errors, which can be triggered by microcode problems. Such errors (machine check events, MCEs) in MC1, i.e., instruction fetch unit (IFU) errors, can happen, when something goes bad in the CPU logic. And my CPU, the Renoir line, was bitten by IFUs while running older AGESA releases (aka, microcode software). Therefore, I updated my BIOS, which also upgraded the AGESA release from 1.2.0.D to 1.2.0.F. Then, I did reset the BIOS to its default values and set the following settings with regard to CPU power management and needed tweaks for my unraid setup:
SVM Mode = Enabled
IOMMU = Enabled
Above 4G Decoding = Enabled
Typical Current Idle = Enabled
Global C-State Control = Disabled
Performance Boost Overdrive (PBO) = Disabled
The following settings would be the optimum, but were not available in my BIOS:
 CPPC Ctrl → Enabled
 CPPC Preferred Cores → Auto
Then, I tweaked the unraid boot settings to this:
append amd_iommu=on iommu=pt amd_pstate=active pcie_acs_override=downstream,multifunction vfio_iommu_type1.allow_unsafe_interrupts=1 video=efifb:off initrd=/bzroot Testing after reboot with dmesg | grep -E "amd_pstate|cpufreq" showed this: amd_pstate: The CPPC feature is supported but currently disabled by the BIOS. Therefore, the amd_pstate did not work, because my BIOS doesn't make this available to the kernel and falls back to acpi_cpufreq, but that is more a nuisance than a problem. Another try with amd_pstate.shared_mem=1 showed the same. Due to this, I ran unraid with this here:
append amd_iommu=on iommu=pt amd_pstate=disable pcie_acs_override=downstream,multifunction vfio_iommu_type1.allow_unsafe_interrupts=1 video=efifb:off initrd=/bzrootWith this setup, I ran 4 passes of memtest for the RAM without any errors, and I can now happily say that the server is up and running without any glitches again...

Hopefully this helps someone else!