Finally solved. I'm going to write the solution in case someone needs this info in the future.
I was attempting to map the tempo by dropping audio clips on the time line. It would go through the process, then give the message "The following VST plug-in failed to load: (80004002) c:\program files\common files\vst3\Celemony\Melodyne". I tried reinstalling Melodyne and Sonar in different configurations, but still ended up with the same message.
I completely uninstalled Melodyne from my machine, scoured ever trace, rebooted, then reinstalled. Still had the same problem.
Used the info on the Cakewalk website to wipe Sonar off my machine. You can find the instructions here:
https://www.cakewalk.com/Support/Knowledge-Base/2007013392/SONAR-2015-Clean-Install-Instructions Reinstalled Sonar Platinum, and it worked. I could use the tempo mapping without the error message.
Now, even before this problem, I was having a lot of sluggish weirdness with Sonar. I'd get crashes on simple projects, and getting larger projects that simply wouldn't load or play. After the clean installation, Sonar is running like a greased pig at the rodeo. No hiccups. No glitches. Projects are loading faster. I hated the trouble, but the solution turned out to be a turn for the better. Very, very happy with things now. I've installed a ton of updates/early releases/etc. in the last couple of years. I guess something wasn't grokking internally. I'm going to put this on the calendar and do a clean reinstall every six months.