Mailbox errors: The HTTP request was forbidden with client authentication scheme 'Anonymous'.

[Polina]

Hi All,

We're actively working with Microsoft to get more details on the issue, and at the same time, preparing a KB article on this issue.

[utleyab]

Same issue here
Case #07899344

[CurtisC]

Same issue here
Case #07898903

[SteelContainer]

Same issue here
Case #03707178

It would be good to get the KB updated with what licenses no longer work and which ones are unaffected. From what I can tell it's mainly the F1 and F3 licenses that are failing.
It also seems some shared mailboxes get the same error, but some are fine, possibly due to EWS being disabled.

[jwilson.aimwa]

Same issue.
Case # 07898123

Our failures are happening on disabled user accounts; they were backing up fine prior to yesterday.

[adam_conlon]

I have logged a ticket for this error with Veeam (#07899918).

I'm getting this error on some unlicensed accounts with shared mailboxes attached (however not all unlicensed shared mailbox accounts, just some)... mutiple tenants, same issue,

[Mohar]

Same issues Case # 07899867
Affecting shared mailboxes and F3 licenses so far.
Shared mailbox issue is not consistent across tenants.

[DHuppertz]

Same issue here for several customers.
Case # 07900045

[Dynamic]

Currently wanted to open also a case for another customer, but regarding this problem, Support portal mentions https://www.veeam.com/kb4796
Do I get it right: Until a couple of days, it was possible to backup those Mailboxes. Now Microsoft changes this in the Background, and the customer has to decide, if he wants to upgrade his corresponding licenses?

[admcomputing]

Resolution is to use a supported M365 license... Thanks...

https://www.veeam.com/kb4796

[aeph]

What?? So thats it? No fix ?

Are you serious? Should we now go to our customers and tell them that we can no longer back up these mailboxes, even though it was still working three days ago?

Is this just temporary information, and will there be a fix for this?

[Roel3960]

So that means we suddenly need to upgrade all our F1 licenses? Nice 1 ... :/

[Seb.pythoud]

same issue here for 2 Tenants over 26 since last night.
case #07900169

[admcomputing]

Going through all logs, we are seeing this error appear for shared mailboxes as well.
Should I raise a support ticket or is Veeam saying these can no longer be backed up and require M365 licenses?

[Zoddo]

We are also experiencing this issue since the last night for about 50 mailboxes.

Is this just temporary information, and will there be a fix for this?

I'm not Veeam, but considering that EWS is being retired on October 1, 2026 (MC1176266), Veeam have to work on another method to backup the mailboxes anyway. I hope this will also fixes the backup of Kiosk/Shared mailboxes.

[Patrik87]

Same issue here to.
Yesterday 2 mailboxes, today 7.

2 of them has Exchange Kiosk Online license - should not work.
Rest of them has F3 licenser activated..
Case number: 07898202

[lmendel]

Also seeing issue for shared mailboxes - can you please confirm we can still backup those? #03827603

[BjoernS]

We have now encountered the error with a customer with groups!?
Case opened with no. 07900260

[f.neumann]

Looks like there's a new KB article for this: https://www.veeam.com/kb4796
But that does not help with groups and shared mailboxes.

[track1044]

The solution on that KB is horrendous. "Spend more money!"
We have around 600+ licenses suddenly unable to be backed up with no solution in sight except spend more money. Not a good look at all.

[rsk@mb-solutions.dk]

Issue has today spread widely across many of our customers now. Hundreds of mailboxes affected!

Case #07893686

[LorisDM]

In my case, currently, only users with an F3 license are affected. I get the same message also when I try to perform a new app registration for the organisation.

Case #07898905

[track1044]

I created a case as well cause I refuse to accept that kb, Case # 07900291
I just redid our scan and we have over 10k affected mailboxes. This is unbelievable.

[ACrispiels]

I'm experiencing the same problem for about 24+ hours now, particularly affecting some SharePoint sites or users with only Teams EEE licenses, or users with Business Basic licenses.
Veeam 365 release 8.1.2.180.
I'm intentionally not opening a support ticket to avoid unnecessarily overloading technical support.

[HenrikITAGIL]

Same issue, with mailbox on E5 license.
Case # 07900424

[DaStivi]

i just noticed issues accessing a shared mailbox through owa... and then i've checked the M365 service status and i just found that there is a issue reported too.. that also might fit into our issues...

https://admin.cloud.microsoft/#/service ... /EX1188132

the start of the issue, according to MS was on 19.11. and i noticed the issues with backup on 18. to 19. so that would also fit perfectly fine...

[Zoddo]

It looks like Microsoft may be rolling back the change.
In my case, an automatic retry that started 15 minutes ago has finally managed to process these F1/F3 mailboxes.

[rsk@mb-solutions.dk]

Yes, all jobs are working now....haha!

[track1044]

Yeah I tried to backup on a client with 162 of these errors and it has gone trough. What the heck is going on here?
Why is the KB there then? Is this an intentional change or a bug?

[Polina]

Hi All,

I can only speculate now, but starting today we also see this issue being self-resolved. And I can only assume that Microsoft has reverted some change on their end.
We'll keep monitoring the situation, and I still hope to get more details from Microsoft. There's no certainty atm that the issue will not reoccur. Veeam's RND will keep researching a longer term solution (if possible) to continue the protection of the impacted mailboxes.