I'm really not sure why I'm bothering with this, but I'll keep adding what I'm doing to try and help myself.
So, I've also now tried enabling "High Sensitivity" scanning, not that that would fix the problem, but at least I'll be extra thorough if and when I actually manage to complete a scan.
I tried disabling config caching at the bottom of the Diagnostics page...but now I don't even see that checkbox at all. Not sure where it went, but I unchecked it after reading this page.
Happened to see that a new version was, coincidentally, released today doing away with config caching. Nice timing to help me think I was really losing my mind. I mean, I'm already talking to myself.