Partner Center: reserving a new app name fails with "Unexpected error" (HTTP 500), no product is created

Since 16 October 2026, I can't create any new MSIX app in Partner Center. In Apps and games → New product → MSIX or PWA app, I enter a name and Check availability says "The name is available". When I click Reserve product name, the dialog shows:

"Unexpected error, we couldn't complete your request. Please try again later."

In the browser's network tab, the request POST /dashboard/product/api/products returns HTTP 500 with {"message":null,"correlationId":"..."}. No product is created, but the name then reads "not available" for a few days, then becomes available again. So every failed attempt temporarily blocks that name.

What I've already tried

• 16 attempts over two weeks, on different days, with different names, including by hand and with a restarted browser. The result is always the same.
• Two reservations succeeded on 16 October (before the failures started), and both apps are now published.
• Creating add-ons for existing apps works. Only creating a new product fails.
• Contacted ReportApp, who replied that they only handle certification/policy questions and directed me to Developer Support. The Engage Center page for support cases doesn't load for me (it shows a spinner).
• There are no "Not started" products left in the account.