Outlook mailbox error SerializationException

Question

I can log into https://account.microsoft.com/ but from there, I click outlook or open outlook in new tab, it goes to something went wrong.

I am not able to access to outlook for like few weeks already. I can sign in to other live account. from there, I click outlook mail and it goes to same error page

Client Version: 20260227002.16 BootResult: fail Back Filled Errors: None err: System.Runtime.Serialization.SerializationException esrc: StartupData et: ServerError estack: Newtonsoft.Json.JsonSerializationException st: 500 ehk: X-OWA-Error efe: BLAPR03CA0067 ewsver: 15.20.9678.19 emsg: ErrorUnexpectedFailure

Answer 1

Hi John Lim,

Welcome to Microsoft Q&A.

Because this issue also occurs in Incognito/InPrivate mode and on a different device (such as a phone), it’s very unlikely to be caused by a local browser cache or extension.

The diagnostics you shared, particularly HTTP 500, SerializationException, and ErrorUnexpectedFailure are commonly associated with Outlook on the web failing server‑side while loading the mailbox startup data. This points away from a client-side problem and more toward how the mailbox is being initialized on the service.

Try this self‑serve step first (Primary alias reset / account re‑handshake)

This doesn’t fix every case, but it has helped in similar mailbox startup failures:

1. Go to Manage how you sign in to Microsoft (alias management).
2. Add a new alias (or use an existing one already on the account).
3. Select Make primary on the new alias.
4. Sign out completely.
5. Sign back in and open: https://outlook.live.com/mail/0/

Important: Do not remove any alias you still need for sign‑in. Removing a Microsoft‑domain alias (Outlook/Hotmail/Live/MSN) can be permanent.

For reference: Add or remove an email alias in Outlook.com

If the error persists

If the mailbox still fails to load after changing the primary alias, the remaining resolution typically requires Outlook.com Support to review and repair the mailbox provisioning on the backend. When contacting them, include the full diagnostics block (for example: st: 500, X‑OWA‑Error, SerializationException, ErrorUnexpectedFailure) to help speed up investigation.

If you’d like, you can also confirm whether Calendar or People fails to open for the same account, that can help distinguish a full mailbox startup issue from a Mail‑only problem.

Hope this helps clarify what’s happening and the best next steps.

Please let me know if this proves useful to you, or if you would like further assistance.

I'm looking forward to your reply.

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