[German]Update KB5066835 from October 14, 2025 (as well as the preview update from September) for Windows 11 24H2-25H2 and Windows Server 2025, comes with a bug. This causes Internet Information Server (IIS) and thus the localhost function to no longer work. This cripples some applications. Microsoft has confirmed the issue and also provided a KIR solution.
localhost issue caused by update KB5066835
In my blog post Windows 11 24H2/25H2: Localhost issues after October 2025 update KB5066835, I addressed the issue for Windows 11 (from version 24H2 onwards). As soon as the cumulative update KB5066835 from October 14, 2025 is installed on Windows 11 24H2 (and 25H2), many applications experience serious problems.
Everything that localhost needs to access Internet Information Server (IIS) no longer works. According to my blog post, this affects numerous applications, from DATEV programs and CRM systems to access management software. In the blog post, I outlined some crude workarounds—or you can uninstall the update to get back to work.
Microsoft confirms IIS problem
On October 16, 2025, Microsoft confirmed the bug described above in the Known Issues section of the Windows 11 25H2 Release Health Status page in the support article IIS websites might fail to load (thanks to the reader for pointing this out). It states that after installing updates (for Windows 11 24H2 – 25H2) on or after September 29, 2025 (which includes the preview update KB5065789 from September 29, 2025, or the cumulative update KB5066835 from October 14, 2025), problems may arise.
Server-side applications based on HTTP.sys may experience problems with incoming connections. As a result, IIS websites may not load and a message such as:
Connection reset –
Error (ERR_CONNECTION_RESET)
or a similar error. This affects websites hosted on http://localhost/ as well as other IIS connections.
Microsoft states that this behavior is caused by various factors that can be influenced by the device's Internet connection, the time of installation of the last update, and the restart of the device. This also explains why the problem does not occur in some environments despite the installation of the above-mentioned update.
Clients running Windows 11 24H2 and 25H2, as well as systems running Windows Server 2025, are affected by this bug. Microsoft is currently working on fixing this bug and plans to correct it in one of the following updates.
Steps to fix the problem
According to Microsoft, the following steps can help resolve this issue in affected Windows 11 environments:
- Simply check for new updates via the Settings page in Windows Update and install them.
- In all cases (even without new updates), Windows 11 should be restarted.
With a little luck, the problem should then be resolved. Restarting is also helpful because Microsoft intends to fix the problem using Known Issue Rollback (KIR). For unmanaged devices (e.g., for private users or in small businesses), restarting helps the KIR solution take effect faster than after 48 hours.
Administrators in companies with managed Windows systems must download, install, and apply the group policy: Windows 11 24H2, Windows 11 25H2 and Windows Server 2025 KB5066835 251015_22001 Known Issue Rollback.msi to enable the KIR function.
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