I've had this problem for a couple of weeks too. Whenever I try to download anything from the iTunes Store (apps, music, whatever), I get the following error message:
I have no idea what "Angel" is and can't find out whether it's an app, a song, a book, a movie, or what. If I try to update all of my apps, I get this once per app, so if I have 40 apps to update, I get 40 message boxes like the one above. If I go into my iTunes account (Store > View Account), it says that I have "1 items awaiting download":
I can't see what item it is and if I click the "Download Now" button, the same error message appears. I've tried to "Reset all warnings for buying and downloading" to see if that might give another, more useful, error message, but alas. I think this error message started appearing after upgrading to iTunes 11.3, but it might be unrelated.