Microsoft is warning that the August 2026 security update for Windows Server 2016 can trigger 0xc0000409 errors on systems where the Compatibility Appraiser diagnostic service is enabled. The issue affects both physical machines and virtual environments, including those running VMware or Microsoft’s Azure cloud platform. On affected servers, users will see the CompatTelRunner.exe process, also known as the Compatibility Appraiser Telemetry Runner, crashing repeatedly.
The Compatibility Appraiser is a background component of Windows Compatibility Telemetry that checks whether a device meets hardware and software requirements for the next major cumulative update or feature upgrade. Microsoft stated that the recurring CompatTelRunner.exe failures generate Application Error events, specifically Event ID 1000 with exception code 0xc0000409, but do not impair device functionality. The company said the event log warnings can be safely dismissed temporarily until a resolution is released.
Microsoft confirmed it is working on a fix that will ship with a future Windows update but has not provided a timeline for a permanent solution. The known issue was detailed in a service alert that was flagged by Microsoft MVP Susan Bradley. The alert lists the originating knowledge base articles for the August update as the trigger for the problem.
This is not the first Windows Server 2016 update problem Microsoft has addressed recently. In June, the company fixed a separate known issue that caused that month’s security updates to fail on systems that were not up to date. The new August issue is distinct but continues a pattern of update-related complications on the aging server platform.
Separately, last week Microsoft warned customers that some Windows Server 2025 systems may experience application crashes due to recent memory management changes. That issue specifically affects applications using Address Windowing Extensions, a set of extensions that allows programs to access more than 4GB of physical memory within a 32-bit virtual address space. On impacted systems, users are seeing memory corruption errors, access violation exceptions with 0xC0000005 error codes, SQL Server crash dumps, and SQL Server services stopping or restarting unexpectedly.
Microsoft has not yet indicated whether the Windows Server 2016 fix will be expedited or delivered on the regular monthly patch cycle. For IT administrators running affected systems, the company’s guidance is to monitor event logs but continue normal operations. The permanent update is expected to arrive in a future release, though no specific month has been named.
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