The Agent Upgrade log (C:\ProgramData\Bit9\ParityAgent\Logs\ParityHostAgentInstall_TIMESTAMP.log) contains:
Failed to grab execution mutex. System error 258.
MainEngineThread is returning 1618
Environment
App Control Console: All Supported Versions
App Control Agent: All Supported Versions
Microsoft Windows: All Supported Versions
Cause
Another installation is taking place at the same time as the upgrade was requested (Rare Event).
Multiple instances of msiexec.exe are running on the endpoint.
Resolution
Reboot the endpoint to be sure pending software installations have completed, and the Windows Installer Service is once again available to be used by the Agent upgrade process.
Additional Information
In some instances the Agent will show as Disconnected due to this failure during the upgrade process, but should become Connected after the reboot.