mailmon.dll is used to interface with the Windows performance monitor software. If you have perfmon, or any other monitoring software that looks at the performance monitor information, open while trying to upgrade your server GMS is unable to update the DLL as it is locked by the operating system.
It is advisable to ensure that all monitoring software is shut down prior to performing an upgrade of your system.
If you do get this error you can manually move the DLL from the gordanoinstall directory at a later time. The file should be copied to your systemrootsystem32 directory. The machine should then be rebooted.
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