The subject error and Registry Editor could not load the key. The file is not a valid registry file. can be caused by the misuse of Save Subtree As.
Save Subtree As exports the selected key and sub-keys as a text file. The file is for human use (view, print, annotate) and can NOT be restored or used to load a hive.
Save Key exports the selected key and sub-keys to a file which can be Restored.
Save Key exports the selected hive to a file. You can subsequently use the file to Load Hive.