I get a report saying 559 files failed to import.
How can I find out which ones and what are the likely causes?
Are you sure that there are no files in unsupported format?
I would import in small chunks by selecting a bunch of folders in Windows Explorer and right-click importing. In my case it would take some time. I seem to have 2318 folders in my library.
This does not probably apply to your situation, but generally it is a good idea to keep several backups of media library. I store about 10 most recent and I make a backup every time before and after bigger changes.
EDIT:
Is it possible to dwongrade to an earlier version of MC11 for which I probably have a back-up library that is not corrupt and still retain my existing licence?
Or, is it possible to extract view schemes and playlist from an earlier back-up?
I don't think the library format has changed after the major database change in MC11.
Registration info is always kept in your system. You can uninstall MC11 totally before reinstalling any MC11 build if you like.
The internal playlists are tied with the library items, not with the file names on the disk. They must be from the same database.
I don't know if the view schemes are included in the backup. If they are, I suppose you could take only the relative files from a backup package.