Microsoft has released emergency, out-of-band updates on Saturday for Windows 10, Windows 11, and Windows Server to fix an issue that prevented Microsoft Outlook classic from opening when using PSTs stored in cloud storage.

Since Microsoft released the January 2026 Patch Tuesday updates, Outlook app users have been encountering freezes when opening the application if a PST file is stored in cloud storage, such as OneDrive and Dropbox.

A Microsoft Outlook PST file is a data file used by the application to store a user’s email and other data locally on their computer instead of on the mail server. In enterprise environments, these files are commonly used to access mail when offline and back up important messages.

“After installing Windows updates released on or after January 13, 2026 (the Originating KBs listed above), some applications might become unresponsive or experience unexpected errors when opening files from or saving files to cloud-backed storage, such as OneDrive or Dropbox,” explains Microsoft.

“For example, in some configurations of Outlook that store PST files on OneDrive, Outlook might become unresponsive and fail to reopen unless its process is terminated in Task Manager, or the system is restarted.”

“In addition, sent emails might not appear in the Sent Items folder, and previously downloaded emails might be downloaded again. The affected Outlook configurations primarily involve classic Outlook, which is commonly tied to enterprise licensing and is not included with most home installations of Windows. To check your Outlook configuration, see Feature comparison between new Outlook and classic Outlook.”

Microsoft releases OOB updates

To address this Outlook problem, Microsoft released the following out-of-band updates today:

• Windows 11 25H2 and 24H2: KB5078127 [Download Link]
• Windows 11 23H2: KB5078132 [Download Link]
• Windows 10 23H2 and 22H2: KB5078129 [Download Link]
• Windows Server 2022: KB5078136 [Download Link]
• Windows Server 2025: KB5078135 [Download Link] 
• Windows Server 2019: KB5078131 [Download Link]

For those affected by this issue, you can install the updates via Windows Update or the Microsoft Download Catalog using the links above.

Last Saturday, Microsoft also released OOB updates for bugs that prevented access to Microsoft 365 Cloud PC sessions and prevented some PCs with Secure Launch enabled from shutting down or entering hibernation.

While previous OOB updates could only be installed manually via the Microsoft Download Catalog, Microsoft says those fixes are now also included in today’s updates.

If these issues do not affect your devices, there is no need to install the out-of-band updates.

Instead, admins and users can wait for the upcoming preview update or next month’s Patch Tuesday, when the fixes will be rolled out as part of the regular update schedule.