I also experienced this message after purchasing several (100+) songs when only half of them downloaded correctly. After 9 days and 3 support reps, they solved my problem. It turns out that there were 2 or 3 songs that caused the problem, but Support remedied the situation by re-adding songs to my account and having me download as little as 7 and as many as 24 at a time. Tedious, but it worked.