Q: an unknown error occurred

Perhaps try the "Error -11556" section in the Specific Conditions and Alert Messages: (Mac OS X / Windows) section of the following document:

iTunes: Advanced iTunes Store troubleshooting

Try opening an elevated command prompt by going to the start screen, then typing "CMD" without quotes and pressing CTRL + SHIFT + ENTER.  It will ask you if you want to allow it to make changes, click yes.

Then, while in the command prompt, you need to type "netsh winsock reset".  You type this without the quotes.  Make SURE you are in an elevated command prompt window before trying this. If you don't know what that means, search google for "elevated command prompt".

b noir above is spot on! Specifically, please go to this page for error 11556 (in most cases, this should solve the error 11556): http://support.apple.com/kb/TS3288

It's that (I found this myself yesterday, after weeks of helplessness in solving the error 11556) whenever iTunes is updated, the Parental Controls in iTunes are, by default, set to 'enabled' mode! One needs to disable/unckeck this on the iTunes menu (under 'Edit' -->  'Preferences' -->  'Parental'). See the relevant discussion/thread here:https://discussions.apple.com/message/22473554#22473554

Regards.

Request the moderator to remove/delete the last sentence in my reply post above (as I posted that sentence by mistake in my reply), i.e., "See the relevant discussion/thread here:https://discussions.apple.com/message/22473554#22473554 "

and replace/substitute the deleted sentence with the foll.:

See the relevant discussion/thread here: https://discussions.apple.com/message/16957408#16957408