I can confirm that Xcode 26.4.1 fixed this for me.

I can confirm that Xcode 26.4.1 fixed this for me too.

I tried dropping back to Xcode 26.3, but it doesn't work there either. Perhaps the bug was actually introduced with a recent MacOS update?

Following up on my previous comment... From the site settings page in Chrome (chrome://settings/content/all), I deleted all site data and permissions for apple.com. I next went back to the Cloudkit dashboard page and refreshed it, but I still got the error message. So I pressed the "Sign out" button there, and then signed back in. It is now working again for me from a regular (non-incognito) Chrome window.

I just started having this problem too. I use Google Chrome for my web browser. I tried it again inside a Chrome Incognito Window (similar to a Safari private window), and it worked fine. Is there some setting or cookie that could be causing this problem?