Important detail: > The only thing Apple Card was paying for was the 2TB iCloud upgrade — and when I try to change the payment method, it refuses and says “Your account has been disabled in the App Store and iTunes.” It's not hard to imagine a payment issue for iCloud causing iCloud to be locked or otherwise limited, and given that Apple's online services tend to be buggy and unreliable it's not a huge stretch to thi…
My mom's Apple account was disabled with the same message, thought it was billing related as well. It turned out that someone with a similar surname had called in and requested their account to be disabled, but the call had cut out. The support rep happily connected the two dots and disabled my mom's account.