This is not surprising, because incorrect application by user generates the error.
Are you aware of the fact, that doing so every file will be touched, even those, where nothing needs to be changed?
The human will is to be sky-reaching.
You can try these two actions ...
No Filter.
Action: Format value
Field: TITLE
Format string: $if($eql($len($regexp($regexp(%ARTIST%,'\s/\s','\a'),'[^\a]',)),1),$regexp(%TITLE%'==='%ARTIST%,'^(.+?)===.+?\s/\s(.+?)$','$1 (feat. $2)'),%TITLE%)
... or ...
Format string: $if($eql($len($regexp($regexp(%ARTIST%,'\s/\s','\a'),'[^\a]',)),1),%TITLE%$regexp(%ARTIST%,'^.+?\s/\s(.+?)$',' (feat. $1)'),%TITLE%)
Action: Format value
Field: ARTIST
Format string: $if($eql($len($regexp($regexp(%ARTIST%,'\s/\s','\a'),'[^\a]',)),1),$regexp(%ARTIST%,'^(.+?)\s/\s.+$','$1'),%ARTIST%)
DD.20120624.1323.CEST
Edit.DD.20120625.1857.CEST