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 – RESOLVED

28/11/2025 08:51:00 AM

NHSmail Reference: INC46693238

Microsoft Reference: EX1189359

Issue Status: RESOLVED

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.

Final Update: 08/12/2025 09:36:00 AM – Microsoft have confirmed that their deployment finished saturating and is now available in the newest version upgrade, 20251128003.06. They have also verified through final customer testing that impact is fully remediated once users upgrade their Outlook clients to version 20251128003.06.

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: A recent change to update how attachments are cached introduced an encoding error within Excel file names which was causing non-ASCII characters to fail, and this resulted in impact.

Next Steps: Microsoft are continuing to review and improve their code change processes to better detect and prevent similar service incidents from occurring in the future.