There is an error with trying to ‘Autocorrect tags from Musicbrainz’ returning error 503 that will only occur for tracks that have no artist, no album and a track no of the form x/y
I think this is what you are encountering, and that the problem is not to do with selecting multiple tracks but to do with thwe tracks that you have selected. If you email support at jthink dot net with your support files using ‘Action/Create Support Files’ I can double check this.
Another thread about this at http://www.jthink.net/jaikozforum/posts/list/15/589.page
If it is this issue it is fixed ready for the next release out before the end of Novemeber.