www.makemkv.com

I have an old Mac utility called RipIt that can extract the DVDs that fail in MakeMKV, but it extracts to files and folders, not to .iso images.

Cool! I guess that's further proof that there is an issue with the MakeMKV Backup function, at least for DVDs, maybe only DVDs using RipGuard?

I'm sure you're aware, but there are utilities to make ISO files from what you were able to extract using your Mac. An old one I still use occasionally is called ISO Buster. I'm sure there are many others and likely many that are completely free.

Usually when I see a problem like this with DVDs I can workaround it by deselecting the subtitle tracks.