It looks like a Yahoo server problem because several people have been asking this question. Hopefully it will be fixed sooner or later. It has been reported by someone on Yahoo's...
2 Answers · Yahoo Products · 24/01/2011
1 Answers · Yahoo Products · 24/01/2011
This was a recent glitch. which many complained about. They were aware of it and were working on it. It was supposed to be fixed but has returned it seems
1 Answers · Yahoo Products · 30/06/2010
Try asking your question on Yahoo Answers