Mac App Store Broken on macOS 10.14 and Earlier

That's the same experience I had using the app store in 10.13 and 10.14 a few years ago. I couldn't log in, and any attempt to download anything would be met with an inscrutable error. Based on these experiences I assumed it had been broken in those versions of macOS for a while.

This would also prevent those users from downloading installers for other versions of macOS.

I've had this for a while now running 10.14.6 Mojave. I'm not certain, but I believe it began before 27 January. I get the message about "data couldn't be read because it is in the wrong format," so I can't sign in, but I can install Mac App Store updates (did one last night).

@vintner links to download macOS installers outside the App Store are easily accessible. I downloaded them several months ago using OCLP. Mr Macintosh has them on his website, too.

I have several Macs running older macOS versions. Filling in a few gaps for the behaviour of App Store:

10.12.6 has the same symptoms as the Apple Support discussion: "An unexpected error occurred while signing in. The data couldn't be read because it isn't in the correct format."

10.11.6 has slightly different wording: "plist parsing error", which suggests it might be a data file compatibility issue rather than a certificate issue.

(I don't see much point testing earlier systems but if anyone cares I still have a Snow Leopard system running somewhere, and I may be able to locate one external drive on which I had every version installed from 10.6 up to 10.10.)

10.13.6 isn't showing buttons at the top so I can't see Purchased, Updates, etc. If I sign out and try to sign in it goes through the password and two-factor then still shows as not being signed in, but no error message.

10.14.6 I have one Mac still signed in, and it can download purchased apps, but I expect that will only last until the session expires. Booting from a clone of that system required sign-in and it fails with the same alert as 10.12.6.

10.15.7 is OK.

I pine for the old days before app stores existed. Thankfully I still rely on apps downloaded or purchased outside the App Store. But it sucks that a single point of unavailability conditions the installation of plenty of apps that still run on older Macs running previous Mac OS versions. I really hope this is just temporary.

@Bri Yes, this has happened several times over the years, and there was a long period where the CSS was broken, but as of early January, at least, it was working again.

@vintner Besides the links here and on Mr. Macintosh’s site, there are also two apps.

@NaOH That sounds like a different behavior than what I saw. Maybe there are two different problems.

@Riccardo Indeed!

The previous problems with older OS versions accessing Apple hosted resources, iTunes, the Mac App Store, iCloud.com, and CSS files served by Apple, were due to the security certificates included with the original OS installers having expired for those servers.

The fix was eventually to download an install a new CA2-G1 certificate from Apple PKI.

Might be worth it for folks experiencing this to check ou the certificte expiration dates in Keychain Access for Apple's server resources.

Looks like it's fixed this morning. I successfully signed in and installed a free app on my old Mini with 10.13, where I couldn't in the last few days.

Like Michel Fortin on 10.13, the issues seem to be resolved on Mojave 10.14.