If you run the exact same command-line again, it works correctly.
The problem is being researched.
NOTE: When you type REG SAVE /?, you receive:
REG SAVE KeyName FileName \[/y\] KeyName ROOTKEY\SubKey ROOTKEY \[ HKLM | HKCU | HKCR | HKU | HKCC \] SubKey The full name of a registry key under the selected ROOTKEY. FileName The name of the disk file to save. If no path is specified, the file is created in the current folder of the calling process. /y Force overwriting the existing file without prompt. Examples: REG SAVE HKLM\Software\MyCo\MyApp AppBkUp.hiv Saves the hive MyApp to the file AppBkUp.hiv in the current folder