Individual developer registration fails at step 4 with status code 400, and no "Apps and games" workspace is created

I am trying to register as an individual developer for the Microsoft Store (country/region: Japan), but the registration cannot be completed.

• Steps 1 to 3 (account type, identity verification, profile details) are completed, and my profile information shows as verified.
• It fails at step 4 (account setup) with "Account creation failed" and the error: "PartnerIntegrationService call failed: Request failed with status code 400"
• Clicking "Retry account setup" gives the same error. I tried again today, several days after the first attempt, with the same result. The first attempt may have shown a 500 error instead.
• During registration, I changed the name in my Microsoft account profile from a nickname to my legal name.
• I can sign in to Partner Center, but only the "My access" workspace is shown. The "Apps and games" workspace does not appear.
• I sent an email to the Microsoft Store support email address but received no reply or confirmation.
• I tried to create a support case through Engage Center (MSA), but the page keeps loading and never displays the form (Edge, Chrome and InPrivate).

How can I complete the registration or reach the right support team?