You have mixed something into the $regexp() function, which might be not allowed ... or does not work per se, ... which is here ... the function $caps2().
You may do what you want in a following second step ...
Action "Format value"
Tag-Field : ARTIST
Formatstring: $regexp(%ARTIST%,'^(.+\sfeat.\s)(.*)$','$1')$regexp($regexp(%ARTIST%,'^(.+\sfeat.\s)(.*)$','$2'),'\b\w*\b','\u$0')
... or ...
Formatstring: $regexp(%ARTIST%,'^(.+\sfeat.\s)(.*)$','$1')$caps2($regexp(%ARTIST%,'^(.+\sfeat.\s)(.*)$','$2'))
From: "SIS feat. eduardo castillo"
To : "SIS feat. Eduardo Castillo"
DD.20151017.1444.CEST