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