M365 only admin locked out MFA error 53003

AADSTS53003 confirms that Conditional Access is blocking token issuance.

If no other administrator or emergency access account can sign in, there is no self-service bypass. Microsoft Support will need to recover access after verifying tenant ownership.

Update the existing support case with the following wording:

"Complete tenant lockout, sole Global Administrator blocked by Conditional Access, AADSTS53003, request Tenant Recovery escalation."

Also include the affected tenant domain, request ID, correlation ID, timestamp, and business impact.

If the Microsoft 365 licenses were purchased through a CSP or reseller, ask that partner to open or escalate the Microsoft support case on your behalf.

After access is restored, create at least two cloud-only emergency access accounts, exclude them from Conditional Access policies, and monitor their use.

More guidance:
https://mstack360.com/enterprise-microsoft-365-security-implementation/