Action "Replace Comment field " doesnt work

I created a user action because i want to remove some fields (this works fine) and replace every existing comment or empty comment fields by a unique string (this only works temporary):
Here the result (in german) i get in the Action window after creating the actions:
Ersetzen "COMMENT": "*" -> "Musik-Archiv"

When i afterwards select all files in a folder and start the action first time the comment fields are changed.
But selecting a new directory and start the same action comment fiels are not changed furtheron.
When i insert the string "Musik-Archiv" manually in the left-hand site comment-select field and press disk symbol (save) the comment fields get changed. But running the action nothing happened to the comment field....??? - Mysterious - isnt it??

Empty fields do not exist. Therefore you cannot "replace" a non-existent contents with something else.
If you want to set certain values into a field then use an action of the type "Format value" or the function Convert>Tag-Tag