Microsoft Graph API /me/todo/lists endpoint consistently returns HTTP 500 or 503

Summary: Microsoft Graph API /me/todo/lists endpoint (and all sub-paths, e.g. /me/todo/lists/{id}/tasks) consistently returns HTTP 500 or 503 with an empty UnknownError body, for a personal Microsoft account, while all other tested Graph endpoints and the To Do web/mobile apps work normally.

Account type: Personal Microsoft account (MSA), consumer authority (https://login.microsoftonline.com/consumers)

App registration: Client ID c7e84c27-aa8a-4dfc-9192-52d851d578a4, delegated permissions Tasks.ReadWrite, MSAL device code flow

Symptoms:

• GET /me/todo/lists → 500, {"error":{"code":"UnknownError","message":"","innerError":{...}}}
• POST /me/todo/lists/{list-id}/tasks → 500 or 503, same empty error body
• Failures observed continuously across a 12+ hour window on 2026-08-06
• GET /me with valid token succeeds normally
• Microsoft To Do web app (todo.microsoft.com) and mobile app work fine with the same account — lists load, manual task creation succeeds

Troubleshooting already completed (all ruled out):

• Forced full re-authentication via device code flow — issue persists with a freshly issued token
• Removed and re-granted app consent at account.live.com/consent/Manage — issue persists
• Confirmed correct scope (Tasks.ReadWrite) is granted and present in the token request
• Tested against a completely different list ID within the same account — same failure
• Confirmed no local network/firewall issue (requests reach Microsoft's edge, response is a valid HTTP error, not a timeout)

Sample request IDs for correlation:

• 9bd021df-739c-4f11-bf3d-7ebccf28dd83
• de60d39c-ebce-41dc-a36e-2d151edfe72a
• 21f345a1-8330-44cf-8767-56a96d2fe0ed