Microsoft is working to resolve a known issue that prevents some users from opening Excel email attachments in the new Outlook client.
According to a service alert (EX1189359) seen by BleepingComputer, the bug has been impacting Exchange Online customers since at least November 23rd.
Microsoft says it has already deployed a fix to address the bug and added that the root cause is an encoding error in Excel file names, which triggers a “Try opening the file again later.” error for affected users.
While Microsoft also noted that this issue may affect any user who attempts to open attachments with non-ASCII characters in the name, it has not yet provided more details on the extent of the problem.
However, this incident has already been tagged as an advisory, a label commonly used to describe service issues typically involving limited scope or 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,” it said when it acknowledged the bug.
“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,” Microsoft added in a Monday update.
The fix has yet to reach all affected customers, and Microsoft advised those experiencing issues opening Excel email attachments to use Outlook on the web or download the file to open the documents on their own systems.
In recent months, Microsoft fixed a major bug that prevented Microsoft 365 users from launching the classic Outlook client on Windows systems and addressed a known issue causing the classic Outlook email client to crash upon launch.
In March, Microsoft addressed a known issue that caused the new Outlook email client to crash when users clicked a button meant to help them switch back to classic Outlook.
Microsoft also announced in January that it would force install the new Outlook on Windows 10 systems starting with the February 2025 security update.
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