Error 400, can't connect to iTunes Store--and can't find Keychain First Aid

I'm not sure when the problem began. Possibly with El Capitan. The iTunes Store displays a pageful of suggested music, but any attempt to actually do anything with the store evokes:

We could not complete your iTunes Store request. An unknown error occurred (400).

There was an error in the iTunes Store. Please try again later.

I'm using Apple's troubleshooting document, If you can’t connect to the iTunes Store - Apple Support

I can access the Internet, I meet the iTunes system requirements, and my OS is up to date (10.11.3),

I downloaded and reinstalled iTunes. Version was 1.3.2.35 before and after installation.

The problem is user account specific. I can reach the iTunes store from another account, a relatively unused "clean" account, but not from "my" main account. This shows that the problem is not anything with any network equipment external to the Mac.

The next step is to try Keychain First Aid, and I'm stumped. It links to an out-of-date page which is inaccurate, and I'm darned if I can find it. I assume I'm supposed to be using the Keychain Access app, which I hardly ever use, and I can't find Keychain First Aid. Typing "First" into the Help box gives no results.