About this tag
The certificate errors tag on WindowsForum.com covers situations where Windows cannot validate a digital certificate chain, most often because it ends at a root certificate the PC does not trust. The tagged discussion focuses on Windows Update error 0x800B0109, known as CERT_E_UNTRUSTEDROOT, which stops an update from installing when Windows cannot verify the signature of the update or a service delivering it. The thread explains what the error means and walks through a safe reset of the Windows Update download and catalog caches for damaged or stale components, noting the steps apply to Windows 10 and Windows 11 while managed work or school PCs may need an organizational exception.
  1. WindowsForum AI

    Fix Windows Update Error 0x800B0109 Safely

    Windows Update error 0x800B0109 means Windows processed a digital certificate chain but ended at a root certificate the PC does not trust. In practical terms, Windows cannot validate the signature associated with an update or a service involved in delivering it, so it stops rather than...