Failed to access mailbox.. An internal server error occurred. The operation failed., Keyset does not exist

[aeph]

Hello everyone,

since a few hours i am getting lots of errors on Exchange Mailboxes and Teams Teams with this kind of errors:

Failed to access mailbox.. An internal server error occurred. The operation failed., Keyset does not exist
OR
Failed to execute backend request.
OR
Directory Concurrency Limit Reached

I raised a support case. Saw this issues on 6+ of my tenants since a few hours.

Any of you having the same issues?

UPDATE: Saw that MS is having an incident at the moment.

Case #08217974

[martin.michaelis]

Hi,

we are seeing the same issues. Case ID #08218356.

Is there something official from MS regarding the incident mentioned above?

[bielekovve]

Hi,
we are also massively affected... since Monday afternoon/evening... I will open a case soon...
"
01.09.2026 19:00:19 :: Processing mailbox failed with error: Failed to access mailbox.. An internal server error occurred. The operation failed. (mailbox: abc@xyz) :: 1:45:45
"
May be or better I am sure it is related to the MS issues EX1464935 and MO1465074...
Kind regards...

[boar]

Same here, but not all

02.09.2026 13:18:44 :: Processing mailbox failed with error: Failed to access mailbox.. An internal server error occurred. The operation failed. (mailbox: .......) :: 0:52:29
02.09.2026 13:18:44 :: Mailbox processed successfully: ....... :: 0:52:29
02.09.2026 13:18:44 :: Processing mailbox failed with error: Failed to access mailbox.. An internal server error occurred. The operation failed. (mailbox: .......) :: 0:52:34

[Polina]

Hi All,

Our engineering is on it. I cannot confirm the root cause yet, but will provide you an update once it's known.

Thanks!

[neilp]

I am also getting these errors. For some MBs, not all.
It looks like it started 8/31/2026

[cw1972]

just to note, there were some major exchange online infrastructure issues around this time frame, could be connected

Issue ID
EX1464935
Affected services
Exchange Online
Status
Post-incident report published
Start time
31 Aug 2026, 15:56 BST
End time
3 Sept 2026, 11:00 BST
Issue type
Incident


User impact

Users may have been unable to connect to, or experienced degraded functionality with Exchange Online.

More info

We've confirmed that the authentication component issue impacted other services beyond Exchange Online. For more information regarding other impact scenarios, please see: MO1465074.

[bielekovve]

Hi, the error messages in our environment ran away as MS fixed their authentication failure... Kind regards...