Azure Entra tenant lockout after converting only administrator from External to Internal

I am locked out of my Azure/Microsoft Entra tenant after accidentally converting my only administrator account from an external user to an internal user.

Affected tenant:

Tenant name: Basic PAYG

• Tenant ID: available privately to Microsoft Support
• Azure subscription ID: available privately to Microsoft Support
• Original administrator account: my personal Microsoft account (details available privately)

Original administrator account: ******@outlook.com

The account originally authenticated through Microsoft account/live.com. After the External → Internal conversion, the original sign-in no longer works.

I do not know the new internal UPN/password that were specified during the conversion, and there is no other administrator in the tenant.

I receive AADSTS50020 and AADSTS16000 errors when attempting to access the tenant.

I have already attempted Microsoft Support by phone. I was redirected to the web support flow, but the Azure Support Portal itself repeatedly produces the AADSTS16000 error, so I cannot create a support request through the portal.

I am requesting assistance with the Microsoft Entra tenant-lockout process and escalation to the Data Protection / Tenant Recovery team.

The account I am using to post this question is a separate account and is NOT a member of the affected tenant.

I can provide tenant ownership/subscription verification and the requested information privately.