Ariel S 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:
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