A couple things to try and why:
1) are you using digital ice?
if so try it without
2) Try a raw scan (no optimization), wide gamut, full bit depth (48?). Open in PS, apply levels (first set the options in levels to:enhance per channel,shadow and highlight clip at .10) select auto.
Much like a a traditional print there appears to be 'blooming' were the extreme highlite and shadow meet (although in a trad print the blooming occures in the highlight). So you would want to turn off anything that would be causing the scanner to make multiple passes such as ice.
There is a smaller chance that the software is 'enhancing' the shadow detail. The raw scan using the widest gamut should catch it.
I have seen this before. My guess is the lens and/or the ccd chip in the scanner has dust on it. You could try blowing it out or take apart the scanner for a clean. Nikon charges about $290. for a complete cleaning and overhaul if you want to go that route. I seem to remember there are DIY instructions to clean a Nikon LS 2000 scanner on the internet somewhere.
It looks like blown out highlights to me, I have had similar things happen. I'm not familiar with your scanner or the software you use but if there is a curve or histagram control in there you can remove the problem by adjusting the contrast. I suspect that you are trying to produce the visualised end result at the scanning stage, my advice is to forget it, remember, you can do a lot of work in PS after the scan.
Think of the scan being the same in digital terms as developing the negative in analogue terms. You would never knowingy overdevelop a film and blow out the highlights would you.
Can't resist this one Eric, if we had a digital forum this question would have been directed there?? Just my mischevious sense of humour as you well know.
I suspect it's the dirty scenario. Sucks. As you get older those darn little screws get harder and harder to see. Just changed a battery in my watch. Use to take me about 5 minutes. Now 30 minutes. 25 minutes being stubborn, and 5 minutes with the reading glasses.
News flash! While I still think the insides of the scanner are a bit dusty, I finally got the Nikon software to work. Guess what? No more blow-out halo type highlights. At least with the slides I tried. I will have to dig out the Xmas Cactus slide again and give it a whirl.
Vuescan is nice, but I think I need to learn how to control it as it appears to be causing this problem.
I will keep you posted, as I'm sure this is the most intensely interesting thread on the board right now. LOL.