Bottom line
So I'm not sure what happened exactly, except that a complete reinstall and manually removing residual MC16 folders fixed it.
Background
I have a desktop PC on which I do all my importing, and tagging. I usually make a library backup on a NAS device, and restore the backup (minus settings) on the HTPC that is giving me trouble.
What I did
So on the afflicted HTPC, I went to:
C:\Users\Joe\AppData\Roaming\J River\
and there was no Media Center 16 folder, only one for Media Center 15.
I found that the only location for MC16 plug-ins was:
C:\Program Files (x86)\J River\Media Center 16\Plugins
I deleted the C:\Program Files (x86)\J River\Media Center 16\Plugins folder in case this was the one to delete.
I used logmein to connect to the desktop PC. I found the (PC) C:\Users\Joe\AppData\Roaming\J River\Media Center 16\Plugins folder as expected. I opened it up and saw all the folders for the various filters. I copied these to (HTPC) C:\Program Files (x86)\J River\Media Center 16\Plugins, in case they'd been deleted because they were not there.
This made no difference whatsoever. I also restored a copy of the desktop library with settings, in case that would help. It didn't.
Next I uninstalled MC16, including the library (I have plenty of backups). I also tracked down all the MC16 folders that were left behind and deleted them too. I'd already uninstalled+reinstalled MC16 before without doing this and it didn't help.
I reinstalled MC16 from scratch and made the bare minimum tweaks to the options to be in accord with my preferences. I restored the PC library without settings.
Voila all worked well!
I wanted to get my customised settings, so I restored from the HTPC library with settings. Total fail - the same error message. So I figured that the library was corrupt. I tried to restore the PC library with settings to overwrite the HTPC library's settings, with the aim of fixing these to work on the HTPC. That didn't work. And no amount of restoring new libraries would fix it.
So, I went back and reinstalled MC16 cleaniy, manually deleting the folders that were nto removed y Windows unistall. I restored the PC library, minus the settings. All worked perfectly. I just had to sacrifice my customised toolbars and maybe a view or two.
I'm guessing that this might have something to do with the bug with the settings being read before implementation, or that something happened to corrupt my library when I updated my Intel drivers. I should have closed MC when I did it. The installers for any program always recommend it, but I was thinking, hey, it's not 2002 any more.