Global Administrator locked out due to MFA setup failure (AADSTS9001026) in new Microsoft 365 tenant

Question

We created a new Microsoft 365 tenant.

The initial Global Administrator account is unable to complete Microsoft Authenticator MFA setup.

When trying to register MFA, the following error occurs and the QR code is never displayed:

AADSTS9001026:

The multiple resource request is not supported over the /organizations or tenant-specific endpoint.

Authenticator setup fails both on iPhone and PC browsers.

Accessing MFA setup via https://aka.ms/mfasetup and https://aka.ms/mysecurityinfo also results in the same error.

Other users can sign in to the tenant, but they cannot be granted admin roles.

The "Add assignment" button for Global Administrator is visible but disabled.

This results in a complete Admin Lockout situation.

Request:

Please advise how to perform tenant-level admin recovery or MFA reset for the Global Administrator account.

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<モデレーター注>
この質問は「Microsoft 365 と Office | Access | ビジネス向け | Windows」に投稿されましたが、Microsoft Q&A(ja-jp)ではサポートしていない質問内容なので「監視対象外」に変更しました。