This error is probably the same as in this thread, and fixed with this fix.
The original errors only happened when downloading an update and no-where else: not on boot, not at random times, just on updates.
Once I was aware of the other issue, monitoring my system showed regular errors when downloading updates to incusos only, exactly as the other issue would cause.
Once the fix was in place (version ‘555) there have been no more errors, and there have been three pushed updated since then.
It’s still possible that dodgy hardware is in play, as well, but it looks good and I’m going to call this fixed.