An extra piece of new info. It turns out the unexpected errors are specific to one kind of keychain called, "Preferred Thread network Record". These items take the same name as the regular keychain item.
For example, I have one keychain for my router, with the name "Router", and the kind is "AirPort network password" . There are three more items with the same name (Router) that are all 'preferred thread network record' kind of keychains. Selecting any of those pops up the "unexpected error".
If one of those exists for a keychain, and I search that keychain, I'll also get the error, as if I had selected the "preferred thread" version. I get a couple of different error details for those.
Otherwise, I don't get the error. Those 'network Record' keychains are sprinkled throughout my keychain list. I don't know where they came from, but something isn't right.
Any ideas? Thanks in advance!