Noticed this today as well. Also find that when I remove an album, then add it again, I need to collapse the the artist and open again before it shows..worse is when I remove/add an artist, I have to collapse the main tree and scroll down to find the addition. A7 showed the addition automatically. However, MM's getting better all the time!
It seems to have added all my music files, but I also have album art as .jpgs and .alb files in that folder. It doesn't crash it or anything, just not sure why it's giving me this error.