Is anyone else having this problem? It's not letting me buy music today. I first tried about 8 hours ago and got the following message: Could not purchase "Unknown." An unknown error occurred (500). There was an error in the Music Store. Please try again later. This morning, I am still having the problem and there doesn't seem to be any knowledge base or helpful Apple support forum posts on error 500. I have tried a number of troubleshooting steps to fix it (trash iTunes prefs, try on different Mac, try on a PC, etc.) to no avail. An error occurs when I try to change my billing info too. Is anyone else seeing this error?