The Windows update released on January 13, 2026, is causing critical issues with applications like Outlook that access cloud storage, including OneDrive and Dropbox.
Users may encounter unresponsive apps or data loss scenarios when attempting to open or save files stored in cloud-synced locations.
The problem was introduced with cumulative update KB5074109, which brought OS build 26100.7623 to supported versions of Windows. Microsoft has confirmed the issue in a bulletin published yesterday. According to it, applications that handle local files and synchronize them with cloud storage may fail to respond properly, particularly when files are actively edited or saved.
One of the most impacted use cases involves Microsoft Outlook, especially when PST (Personal Storage Table) files are stored within OneDrive directories. In such scenarios, Outlook can hang indefinitely or fail to relaunch after closing. Users have also reported problems with sent messages not appearing in the Sent Items folder and emails being re-downloaded, suggesting inconsistencies in local mail data handling.
The issue appears to be rooted in how certain cloud services interact with file locking and synchronization mechanisms introduced in the update. While Microsoft has not published a full technical breakdown, the behavior suggests possible deadlocks or file access errors when applications try to modify or read files that are being managed by cloud sync clients simultaneously.
The impact spans a wide range of platforms:
- Client: Windows 11 (versions 25H2, 24H2, and 23H2), Windows 10 (version 22H2), and Windows 10 Enterprise LTSC (2019 and 2021)
- Server: Windows Server 2025, version 23H2, Server 2022, and Server 2019
These platforms are widely deployed across both consumer and enterprise environments, raising concerns for IT departments managing cloud-integrated workflows.
As a temporary workaround, Microsoft recommends removing PST files from OneDrive and storing them locally to avoid sync-related interruptions. A support article is available for users needing guidance on detaching Outlook data files from OneDrive-managed folders. Additionally, affected users may access their mail through webmail interfaces, which remain functional and unaffected by the issue.
There is currently no official patch or timeline for a fix. Microsoft states that it is actively working on a resolution and will provide further updates as development progresses. Enterprise customers facing urgent disruptions are advised to contact Microsoft Support for Business for interim assistance.
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