Store developer onboarding (company account): "Send code" for contact email verification always fails with "timeout of 25000ms exceeded"

We are onboarding a company developer account through https://storedeveloper.microsoft.com (new onboarding flow, Company account type). The business details step succeeded: our D-U-N-S number was found and validated automatically.

On the "Contact details" step, clicking "Send code" for the verification email always fails after about 25 seconds with the error "timeout of 25000ms exceeded". No email is ever sent: our mail server logs show no delivery attempt from Microsoft during any of these attempts, while email from other providers (Apple, Google, Stripe) reaches the same addresses within seconds.

What we tried:

• several addresses on our company domain, all deliverable
• retries spread over several hours, in two different browsers
• opening a Partner Center support ticket: not possible, the ticket form requires signing in to Partner Center, which only accepts work or school accounts, while the Store onboarding uses our personal Microsoft account (MSA)
• the Microsoft Edge Add-ons enrollment page also answers "We couldn't find an account with that username" for the same MSA, although the account exists and works on account.microsoft.com

Could the Store / Partner Center team check the verification email service for this onboarding, or tell us how a company onboarding with an MSA can open a support case?

Company: WARRENBROWSE SRL (Romania).