Microsoft says August 2026 update may trigger 0xc0000409 errors on Windows Server 2016

Microsoft says the August 2026 security update may trigger 0xc0000409 errors on Windows Server 2016 systems with the Compatibility Appraiser diagnostic service enabled.

On affected systems, the CompatTelRunner.exe process, also known as Compatibility Appraiser Telemetry Runner, crashes repeatedly. Microsoft Compatibility Appraiser is a Windows Compatibility Telemetry component and background task that checks whether a device meets the hardware and software requirements for the next major Windows cumulative update or feature upgrade.

The known issue affects physical devices and virtual machines, including VMware and Azure environments. Microsoft says affected systems may generate recurring Application Error events with Event ID 1000 and exception code 0xc0000409 associated with CompatTelRunner.exe.

Microsoft says the failures do not affect device functionality. The related event log warnings can be safely dismissed temporarily while Microsoft prepares a fix for a future Windows update. The company has not shared a timeline for a permanent solution.