How to uninstall Windows update in case of ERR_HTTP2_PROTOCOL_ERROR

Learn how to remove a Windows Update when your Ozeki SMS gateway site — https://localhost:9515 - fails to load. This guide walks you through safe methods to uninstall problematic Windows update. Follow these steps to restore normal connectivity and system stability.

For a deeper understanding of the ERR_HTTP2_PROTOCOL_ERROR linked to the KB5066835 Windows update, you can explore the detailed discussion on Microsoft Q&A.
https://learn.microsoft.com/en-us/answers/questions/5585563/localhost-not-working-anymore-after-2025-10-cumula
The page features firsthand reports from developers and IT professionals who experienced localhost failures after installing the October 2025 cumulative update. It includes troubleshooting suggestions, confirmation that uninstalling KB5066835 resolves the issue for many users, and ongoing conversations about potential fixes and workarounds. This resource is especially valuable for those managing local development environments or business applications affected by the update.

The Figure 1 image displays the error message encountered when attempting to access the Ozeki SMS gateway via localhost. The ERR_HTTP2_PROTOCOL_ERROR indicates a disruption likely caused by a recent Windows update, specifically KB5066835, which interferes with HTTP/2 communication protocols.

ERR_HTTP2_PROTOCOL_ERROR
Figure 1 - ERR_HTTP2_PROTOCOL_ERROR

To begin resolving the issue, open the Windows Settings menu by clicking the Start button and selecting the gear icon. This central hub allows access to system configurations, including update management. (Figure 2)

Open Windows settings menu
Figure 2 - Open Windows settings menu

Within the Settings interface, navigate to the “Windows Update” section. This menu provides options for checking, managing, and troubleshooting updates installed on your system. (Figure 3)

Select Windows Update menu
Figure 3 - Select Windows Update menu

Click on “Update history” to view a chronological list of updates applied to your system. This helps identify whether KB5066835 has been installed and confirms its presence before proceeding with removal. (Figure 4)

Check Update history
Figure 4 - Check Update history

From the Update History screen, select “Uninstall updates.” This action opens a control panel window listing all removable updates, allowing you to target and remove problematic ones. (Figure 5)

Open Uninstall updates menu
Figure 5 - Open Uninstall updates menu

Locate the KB5066835 update in the list and select it. Click “Uninstall” to initiate the removal process. This step is crucial to eliminate the source of the HTTP/2 error and restore normal network functionality. (Figure 6)

Uninstall KB5066835 update
Figure 6 - Uninstall KB5066835 update

After uninstalling the update, restart your computer to apply the changes. A reboot ensures that the system reconfigures itself without the influence of the removed update. (Figure 7)

Restart Computer
Figure 7 - Restart Computer

Finally, reopen the localhost page to verify that the ERR_HTTP2_PROTOCOL_ERROR no longer appears. If the page loads successfully, the issue has been resolved and your Ozeki SMS gateway should function normally. (Figure 8)

Open localhost page
Figure 8 - Open localhost page

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