Hello @J. N. Seebeck,
Thank you for reaching out on Microsoft Q&A.
Based on your description, I understand that User A is unable to sign in to office.com or other Microsoft services due to Error Code 530031.
The error is being caused by a Conditional Access policy that is blocking access token issuance due to a matching policy.
Upon reviewing the Conditional Access policies applied to Group A, I see that users in this group are required to sign in from either an Intune-compliant device or a domain-joined device.
Since User A's device is unregistered, it is not being recognized as compliant or domain-joined, which is why the Conditional Access policy is preventing token issuance and blocking sign-in.
To resolve this issue, please ensure the device is properly registered with Microsoft Entra ID. Depending on whether this is a personal or work device, the following guidance will assist in verifying and completing the registration process:
For personal devices: https://support.microsoft.com/en-us/account-billing/register-your-personal-device-on-your-work-or-school-network-8803dd61-a613-45e3-ae6c-bd1ab25bf8a8
For work devices: https://support.microsoft.com/en-us/account-billing/join-your-work-device-to-your-work-or-school-network-ef4d6adb-5095-4e51-829e-5457430f3973
Once the device is successfully registered, please test the sign-in process for User A again.
If the issue persists, feel free to provide additional details, and I’d be happy to assist further.
I hope this information is helpful. Please feel free to reach out if you have any further questions. If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Thanks,
Chaithra.