I think I found a solution in 3 steps:
1.) Split your existing path into a temporary tag field, like MY_SPLITTED_FOLDERPATH with an
Action Type "Format value"
Field:
MY_SPLITTED_FOLDERPATH
Format string:
$replace(%_FOLDERPATH%,'\','\\')
this would look like this if you press ALT + T:
2.) Get the needed item from the newly created MY_SPLITTED_FOLDERPATH tag into the ARTIST tag with a second action:
Action Type "Format value"
Field:
ARTIST
Format string:
$meta(MY_SPLITTED_FOLDERPATH,3)
The number after the comma indicates the item number. In our example
C: would be item number 0
Temp would be item number 1
mp3-Test would be item number 2
author would be item number 3
book would be item number 4
3.) Clean up all the no more used MY_SPLITTED_FOLDERPATH tag fields with an action
Action Type "Remove fields"
Fields to remove:
MY_SPLITTED_FOLDERPATH
Thanks go to DetlevD for his idea.