I do not think that the backslash will be needed.
Now: load all the files that need treatment:
_FILENAME describes the current filename, the focus is on a single file.
_DIRECTORY describes the folder in which the current file resides.
_PATH describes the full path towards the file. This is not needed here.
So, to rename any folder you have the choice:
either enter a relative folder name - this starts at the folder of the current file but excludes the filename -
or enter a fully qualified folder name with all the bits and pieces like drive letter, starting directory.
The fully qualified folder name enables you to move complete folders with all the files in them freely about in the filesystem.
Coming back to your post:
To rename the current folder "A Night At The Opera" to "A Night At The Opera (320CBR)" and the latter data happens to be the data in ALBUM then use an action:
Format value for _DIRECTORY
Format string: %album%
There should not be any backslashes required.