I've helped several friends and family members lately with various Apple-related support issues. As often is the case, however, I had been neglecting one of my own. One of the Macs in my house had been doing that thing where iTunes repeatedly prompts for passwords, to sign in again, or even to sign out and sign back in. And these prompts would appear for multiple iTunes services (automatic downloads, iTunes Match, etc.). So firing up iTunes resulted in several of these prompts - one after another after another. Ugh.
Finally, three things collided: getting the prompt, having a few minutes to spare, and this Apple support article landing in my RSS feed. For me, removing the SC Info folder did the trick.