First: *THANKS*
Second: FYI: SysNative Forum has enough rank to appear in my SERPs for "System Protection" 0x81000203
Third:
I have 2 very similar W10 PCs, both running Kaspersky IS2020. On PC#1, I created a Kaspersky Rescue Disk (USB) which is sufficient for both; i.e., I did not do it on PC#2 - this was the only difference. PC#1 (not #2) developed the problem in this topic; so ...
On PC#2, I started Regedit, drilled down to the above key, and found
"UpperFilters REG_MULTI_SZ klbackupdisk volsnap fltsrv"
On PC#1, "volsnap" was missing, so I double-clicked the UpperFilter and added "volsnap" to make it match.
After restart, System Protection is fixed. I don't know if other Kaspersky features might cause the subject problem, but I'm certain my version was caused by creating the KRD. Now I just need to go pester Kaspersky.
