New outlook cannot save attachment to "download file" with error Code 400

Question

New outlook cannot save attachment to "download file" with error Code 400

Patrick-1601 10

When I pressed the save all attachment on New outlook, it keeps running for a while then come up with a error screen with code 400, then outlook is hanged.

I tried to reinstall outlook again but problem still exist.

Pls help & advice on solutions.

Thanks !

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2 answers

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Answer 1

Demi Nguyen 1,505 Independent Advisor

Hello Patrick,

To help narrow down the cause, could you please confirm whether this error occurs with attachments from only one specific email or from all emails? Please also try downloading each attachment individually instead of selecting Save all attachments, and let me know whether the individual files can be downloaded successfully.

As an additional comparison, please sign in to Outlook on the web with the same account and try downloading the same attachments. Please confirm whether error code 400 also appears there.

In the meantime, please try Repair Outlook:

Sign out of New Outlook, then close the application completely.
Right-click Start and select Installed apps.
Find Outlook in the application list.
Select the three-dot menu next to Outlook, then select Advanced options.
Select Repair.
After the repair is complete, restart the computer, sign back in to Outlook, and test the attachment download again.

If the issue persists, then select Reset. Please note that resetting the application may remove its locally stored app data and settings, so you may need to sign in and configure the account again afterward. User's image

You can also try clearing the web cache used by New Outlook:

Press Windows + R to open the Run dialog.
Enter: olk.exe --devtools

User's image

This opens the new Outlook on Windows client and an instance of the Microsoft Edge DevTools.
In the Microsoft Edge DevTools window, select the Network tab. Select and hold (or right-click) anywhere in the Requests table. Then, select "Clear browser cache".

The 'Clear browser cache' option is selected in Microsoft Edge DevTools.

You can follow the instructions provided in the Microsoft article below: Clear the Office cache - Office Add-ins | Microsoft Learn

Please update me with the results, particularly whether individual attachments can be downloaded and whether the same error occurs in Outlook on the web. This information will help me understand the scope of the issue and assist you more effectively.

Kind regards,

Patrick-1601 10 Reputation points

2026-08-27T09:14:43.8766667+00:00

Hi Demi,

Thanks for your help.

The problem doesn't exist on web mail;

The problem exist on attachment from all parties.

Will try your advices and revert back on the reslut.

Have a good day

Rgds
Demi Nguyen 1,505 Reputation points Independent Advisor

2026-08-27T10:50:57.9+00:00

Hi Patrick,

Thank you for the update. Please try the suggested steps when you have time and let me know how it goes.

Kind regards,
Patrick-1601 10 Reputation points

2026-08-28T02:52:42.2633333+00:00

Hi Demi,

with regret the problem still there after tried all relevant steps.

Thanks !
Demi Nguyen 1,505 Reputation points Independent Advisor

2026-08-28T13:08:58.8133333+00:00
Hi Patrick-1601,

Thank you for the update. I’m sorry to hear that the issue remains after completing the previous troubleshooting steps.

As an additional check, could you please verify the TLS settings on the affected computer?

Press Windows + R to open the Run dialog.

Enter inetcpl.cpl, and then select OK to open Internet Properties.

Go to the Advanced tab and scroll down to the Security section.

Make sure that "Use TLS 1.2" is enabled.

If "Use TLS 1.3" is available, please enable it as well.

Select OK.

Restart the computer, open Outlook, and test the attachment download again.

Please let me know whether these options were already enabled and whether the behavior changes after restarting the computer.

Kind regards,
Arko 0 Reputation points

2026-09-03T03:16:56.37+00:00

I had the same issue but was able to solve it by reinstalling Outlook. Before that, I had tried all the steps given by Demi Ngunyen. So I uninstalled Outlook via installed apps, then reinstalled it via the Microsoft Store.
Demi Nguyen 1,505 Reputation points Independent Advisor

2026-09-03T09:03:59.56+00:00

Hi Arko,

Thank you for sharing the solution that worked for you. Reinstalling Outlook through the Microsoft Store may also be helpful for the original poster and other users experiencing the same problem.

I appreciate your contribution to the discussion.
Glen 0 Reputation points

2026-09-09T02:04:02.7066667+00:00

We are experiencing an issue with New Outlook for Windows where multiple users are unable to use the “Save all attachments” function.

When a user opens an email containing multiple attachments and selects Save all attachments, New Outlook attempts to download the attachments but eventually displays an HTTP ERROR 400 page.

The issue is reproducible for multiple users, so it does not appear to be isolated to a single Windows profile or mailbox.

We have also tested the same scenario using Outlook on the Web (OWA). The same users can successfully download all attachments from OWA without any issue. Therefore, the issue appears to be specific to New Outlook for Windows.

Environment observed in the collected New Outlook log:

New Outlook version: 1.2026.818.100

OWA client version: 20260821009.12

WebView2 version: 152.0.4191.66

Account type: Microsoft 365 Business / Entra ID

Affected users: Multiple users

The New Outlook log shows that, immediately after selecting Save all attachments, New Outlook navigates to the following bulk attachment download endpoint:

https://attachments.office.net/.../service.svc/s/GetAllAttachmentsAsZip

This directly corresponds to the Save All Attachments operation.

More importantly, the generated request contains the following parameter:

X-OWA-CANARY=X-OWA-CANARY_cookie_is_null_or_empty

The log records this at approximately 08:32:32.956, followed by WebView navigation to the same URL.

Based on the log, it appears that New Outlook is unable to obtain or pass a valid X-OWA-CANARY value when initiating the GetAllAttachmentsAsZip request. The resulting request then leads to the HTTP 400 page.

Authentication in general appears to be working correctly. For example, New Outlook successfully obtains credentials for https://outlook.office.com/, and the authentication telemetry reports successful token acquisition. Other Microsoft Graph authentication requests during the same session are also successful.

The behavior can therefore be summarized as:

New Outlook for Windows:

Email with multiple attachments → Save all attachments → GetAllAttachmentsAsZip → X-OWA-CANARY is null/empty → HTTP 400

Outlook on the Web:

Same user / same mailbox / same scenario → Download all attachments → Successful

Since this affects multiple users while OWA continues to work normally, we suspect this may be a New Outlook client/WebView integration issue or regression affecting the bulk attachment download flow, rather than an Exchange Online mailbox or individual user profile issue.

Could you please help us confirm:

Whether this is a known issue or regression in New Outlook for Windows, particularly around version 1.2026.818.100.

Why New Outlook is generating the GetAllAttachmentsAsZip request with X-OWA-CANARY_cookie_is_null_or_empty.

Whether there is a known fix, newer build, configuration change, or workaround available.

Whether additional New Outlook diagnostic logs, network traces, correlation IDs, or reproduction data would be helpful for escalation to the New Outlook engineering team.

We can provide the New Outlook diagnostic log, screenshot of the HTTP 400 error, affected user examples, and additional reproduction information if required.

Thank you for your assistance. We would appreciate it if this case could be reviewed as a potential New Outlook product issue, given that it is reproducible across multiple users while the equivalent OWA functionality works correctly.

Best regards,

Glen

Answer 2

Glen 0

Tried all relevant steps including reinstall new outlook but no good.

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