Thanks, Chris. Like I said, everything seems to be working since reinstalling 8. I didn't get any errors when I copied my plugins back into the new install. I'll reply back to this topic if I have any more similar troubles.
I received a PM from someone who explained that the errors I was getting can occur when SU is closed unexpectedly while Saving Toolbar Locations is in process, or when a bunch of plugins with toolbars are closed all at once - I guess by unloading them out of the directory(?). It's possible that the first is what happened to me today. 8 does seem picky and even a little sluggish on start-up and shut-down so maybe it was cranking away on something and I got impatient. Wouldn't be the first time. 😳