[German]Die Oktober 2025-Updates für Windows scheinen einen negativen Effekt auf die Vorschau von Dateien im Explorer zu haben. Mir liegen Berichte vor, dass PDF-Dokumente oder andere Dokumentdateien nicht mehr in der Vorschau geöffnet werden – speziell, wenn sie auf Netzwerkfreigaben liegen.
First reader reports
The first reports that something might have happened to file previews after installing the October 2025 security update can be found in various comments within my German blog.
Warning about dangerous file previews
In this German comment by Mick on the German article Patchday: Windows 10/11 Updates (14. Oktober 2025), he writes that after the October 2025 update, when viewing the PDF preview with Adobe Acrobat Reader, the warning "The file you are attempting to preview could harm your computer" appears in Windows Explorer. However, not all files are affected. The problem is resolved after uninstalling the October update. However, Mike does not specify which version of Windows he is using.
Noch eine Warnung durch einen Leser
German blog reader M.T. also commented on the article Patchday: Windows 10/11 Updates (14. Oktober 2025) and confirmed the same problem. The reader states that the file preview in Windows Explorer no longer works for some users, regardless of the file type. A warning message appears stating, "The file you are trying to preview may damage your computer…" and the preview is denied.
In this context, I had assumed that the warning was due to the latest Office vulnerabilities, where the preview feature was being misused.
Meldungen auf Administrator.de
In this context, I came across this German thread on German site administrator.de, where an IT employee blames the KB5066835 update for Windows 11 24H2-25H2. He wrote that the update in question (at least for network drives) prevents previews in the Explorer preview window for many file types, e.g., PDFs, Excel, and Word documents. After uninstalling the update, file previews work again. A second German thread on administrator.de refers explicitly to PDF previews.
It affects several Windows versions
On October 19, 2025, German blog reader David S. sent an email reporting similar problems with previews, but on different versions of Windows. He wrote, "Strangely, no one has complained yet that the monthly update has caused file previews to stop working for some items, at least on Windows 11 and Windows Server 2019."
David sent me the above screenshot of the warning (thanks). This warning occurs with Office and PDF document files, as the reader has tested—graphic files, on the other hand, work fine.
David reported that a colleague of his was able to narrow down the problem. Files with the MoW flag (Mark-of-the-Web) set are affected. This includes Internet downloads or files on network shares.
The reader wrote that it could be circumvented by calling up the storage location via FQDN. However, he stated that this would not help end users who open the file server via a drive letter.
Discussion and workarounds
On October 17, 2025, Werner already pointed to the discussion Windows 11 October 2025 Update Breaks File Explorer Previews in the Windows forum in this German comment. The post in question contains tips on how to mitigate the problem without uninstalling the October 2025 update responsible.
In most cases, it should be sufficient to add the network shares to the Local Intranet zone or Trusted Sites in the Internet Properties. German blog reader Werner has confirmed that this works and has also provided the appropriate group policies.
Article series:
Windows: File preview in Explorer broken after October 2025 update – Part 1
Windows file preview broken with Oct. 2025 update: Microsoft explains why it's by design – Part 2
Similar articles:
Microsoft Security Update Summary (October 14, 2025)
Patchday: Windows 10/11 Updates (October 14, 2025)
Patchday: Windows Server-Updates (October 14, 2025)
Patchday: Microsoft Office Updates (October 14, 2025)
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