I'd say it was probably IBM's system as they are prone to being glitchy, at least the Starbucks POSs, this is based on experience. But who knows if they will release the details.
From that article: "Square and Starbucks are not totally splitting up. Square said it still processes debit and credit card payments at 7,000 of the coffee chain’s U.S. stores."
I was wondering this too. I'm curious if starbucks will release any additional details other than, we've fixed it.
Some Starbucks stores use Micros; others use IBM hardware and Agilysys software.