Microsoft confirms the new Outlook has an Excel attachment problem

The new version of Outlook has not proved as popular as Microsoft would have liked, and the company has just been forced to admit to another problem with the email client.

Microsoft has published a warning about a problem with open Excel file attachments, noting that the issue has been around for about two weeks. The cause appears to be the use of non-ASCII characters in file names, and a fix is in the works.

As reported by Bleeping Computer, Microsoft has posted a service alert with the ID EX1189359. The alert acknowledges that: “Any user may be unable to open Excel files attached to email messages in the new Outlook client if the attachment contains non-ASCII characters”.

Microsoft also says:

We've developed a fix to address the missing encoding in the requests used to open files. We're validating this deployment while we work to understand why this encoding error is occurring.

The problem has affected large institutions, including the NHS in the UK. The health service posted the following:

Microsoft 365 Alert – Service Degradation – Exchange Online – Users may be unable to open Excel files attached to email messages in new Outlook and receive an error – ONGOING

28/11/2025 08:51:00 AM

NHSmail Reference: INC46693238

Microsoft Reference: EX1189359

Issue Status: ONGOING

Issue Description: Users may be unable to open Excel files attached to email messages in new Outlook and receive an error

More info: Users receive an error stating: “Try opening the file again later.”

This issue impacts attachments that contain non-ASCII characters. As a way to avoid impact, users can utilize Outlook on the web or download the file locally to manually open Excel files.

Current Update: 02/12/2025 09:15:00 AM – The process of validating the fix to address this issue is ongoing. Microsoft anticipate that the deployment of the fix should begin this week, and they will provide an estimation for the completion of this process when available.

Scope of impact: Any user may be unable to open Excel files attached to email messages in the new Outlook client if the attachment contains non-ASCII characters.

Root cause: An encoding error within Excel file names is resulting in this impact.

Next update by: Thursday, December 4, 2025, at 8:00 PM UTC

While Microsoft says that it has developed a fix, there is no word on when it will be rolled out to users affected by the problem.

Image credit: Alexey Novikov / Dreamstime.com