Microsoft Store individual developer registration always fails with error 400 (PartnerIntegrationService)

Hello,

I am trying to register as an individual developer on the Microsoft Store to publish my Windows desktop application.

Every time I submit the registration form, I get this error:

"PartnerIntegrationService call failed: Request failed with status code 400"

What I have already tried:

1. Several different Microsoft accounts, including brand new ones. Same error every time.
2. Checking every field: phone number in international format, no accents or special characters, region filled in.
3. My town (in Switzerland) is not in the city list, so I selected "other city" and typed it manually. Same error.
4. Trying again on different days.
5. Opening a support case from the developer support page, but the Engage Center page never finishes loading.

Country: Switzerland.

Is there a known issue with individual registration from Switzerland, or a specific field that causes this error? How can I complete my registration or reach someone who can check it?

Thank you for your help.