Microsoft says the August 2026 security update may trigger 0xc0000409 errors on Windows Server 2016 systems where the Compatibility Appraiser diagnostic service is enabled.
On affected systems, users will see the CompatTelRunner.exe (Compatibility Appraiser Telemetry Runner) process crashing.
Microsoft Compatibility Appraiser, which controls CompatTelRunner and is a Windows Compatibility Telemetry component, is a background system task that checks if the device meets the hardware and software requirements for the next major Windows cumulative update or feature upgrade.
According to Microsoft, this known issue impacts both physical devices and virtual machines, including VMware and Azure environments.
"After installing the August 2026 Windows security update (the Originating KBs listed above), some Windows Server 2016 devices might generate recurring Application Error events (Event ID 1000, with exception code 0xc0000409) associated with CompatTelRunner.exe," Microsoft explained in a service alert spotted by Microsoft MVP Susan Bradley.
