Kirk,
Unfortunately I haven't been able to reproduce the problem. I deleted all of my catalogs and started from scratch. Now scanning images appears to work for new catalogs. If it happens again, I'll post.
Just for documentation purposes...the message appeared in the Scan Status window. Normally it says "Found: [folder path]" for each image. This message said "Skipping: [folder path] because it doesn't not exisit" for the entire folder. One might expect a "Skipping" status message for folders to exclude, but obviously not folders to scan (unless there was a bug).