I've found a way to fix the problem consistently. The files that won't play were of the [filetype] = dvd. First I had to change all of them to ifo. Then, I have to manually go into the VIDEO_TS folder and change the extension from .dvd to .ifo.
Just doing one of the steps doesn't work. I have no idea when this started to happen, but it was fairly recent or in late MC16, I guess. I now don't believe I did anything to precipitate this, unless it was done long ago and didn't cause playback to break at the time.
This is what I was trying to describe in my post above. I thought you had somehow deleted the .ifo files and had some other problem.
After switching from v15 to 16, this started happening to me also. I realized some of my movies still had .ifo files in their directories, and these would play. But most of the .ifo files had been changed to the .dvd extension and would not play. For some reason, I thought MC was forcing Win XP to display these .ifo extensions with an .dvd alias (silly, I know) when instead, it had simply changed the filenames to the .dvd extension.
Again, what did it for me was starting with an empty library (obviously not necessary) and somehow triggering a mass name change that put the file extensions back to the .ifo type.
Since that library re-build I always Import the ripped movies first, then tag them using Fill Properties From File Names, and I haven't had the problem since.
I realize now that my first post in this thread was misleading. The problem actually appears to be the actual extension name change and what triggers it.
My experience with this involves:
1) Inadvertently triggering the MC program to change .ifo extensions to .dvd extensions when importing movies into MC16 (this could be a bug, or an easily made operator error of some sort).
2) The second unplanned mass filename change that put my .dvd extensions back to .ifo extensions.
The thing that leaves me baffled is how I triggered the problem and the fix. I haven't tried to use MC's Rename, Move, & Copy Files function ever...