iTunes can't verify server identity: error message

A few days ago this message started popping up every time I opened iTunes, and then periodically while I'm in it, even if I'm just listening to my own music and not browsing the store:

iTunes can’t verify the identity of the server “p26-buy.itunes.apple.com”.

I was able to find a Keychain-related post via Google, but creating a new keychain from the old ones didn't help.

Has anyone else encountered this and fixed it? Repair permissions didn't do anything either, and in El Capitan "repair keychain" is no longer an option.

Kirby