Feedback filed: FB24075397 I really appreciate your efforts to help!

No, you've had it right. macOS 26 host, 26 and 27 beta guests.

I just ran through it with a macOS 26.6 guest on UTM. Same outcome. I've looked in the past and not found anything obvious in Console.app. Is there a debug profile I can install that will shine more light on the failure?

I installed a brand new Tart VM from the 26.6 .ipsw, 26.6 host, still cannot sign into my Apple account. I changed the VM name to "Tahoe VM" and it still fails with verification failed/unknown error result. I tried a second Apple account that also has 2FA enabled and it failed with the same error. Same experience with two different macOS hosts.

A fresh install of 27.0 beta 4 on a 26.6 host results in the same problem. I tried to sign in as part of the VM install and through System Settings, with the same verification failed/unknown error result.

Yes, this account has a trusted phone number and I get the 2FA prompt when I sign into it in Safari.

The host is macOS 26.5. The guest is currently running 27.0 b1. I'm using Tart as the VM app. Initially I wasn't able to install the 27.0 beta due to the known incompatibilities between the 27.0 beta ipsw and macOS < 26.6, so I created this VM image by installing 27.0 beta on a spare Mac, then creating the VM there and moving it back to my production Macs.