Q: An unknown error occurred (-50)

Perhaps try the "Error -50," "-5000," "8003," "8008," or "-42023" section in the Specific Conditions and Alert Messages: (Mac OS X / Windows) section of the following document:

iTunes: Advanced iTunes Store troubleshooting

An unknown error (-50) usually occurs when you have a connection issue with the iTunes Store during download.

This article will help you out to fix it.  Follow it as best you can!

iTunes Store: "Error (-50)" when downloading purchased content