I cannot say anything about the issue between scrwfile and titletoc other than Markus Kohm back in past frequently uttered about possible incompatibilities with such programs, that change KOMA-script class commands. I actually do not know, whether it is the case here. Regarding to titletoc’s “sister” package I wrote something about this a while ago, there may meanwhile have been changes for compatibility, though: Incompatibilities between KOMA-Script and titlesec.
But I can suggest an alternative: minitoc.
\documentclass{scrbook}
\usepackage{scrwfile}
\usepackage[nohints]{minitoc}
\mtcsetrules{parttoc}{off}
\mtcsettitle{parttoc}{Part Contents}
\mtcsettitlefont{parttoc}{\LARGE\bfseries\sffamily} % "\sffamily" is KOMA-script default
\newcommand{\setparttoc}[1]{%
\mtcsetdepth{parttoc}{#1} % "0" as usual chapter depth
\parttoc
}
\begin{document}
\doparttoc
\mainmatter
\tableofcontents
\part{Part Heading First}
\setparttoc{0}
\chapter{First Chapter}
\section{First section}
\chapter{Second Chapter}
\section{First section, again}
\part{Part Heading Second}
\setparttoc{1}
\chapter{Chapter One}
\section{Another first section}
\chapter{Chapter Two}
\section{Why first sections only?}
\end{document}
Output of example in question (with scrwfile deactivated):

Output of example above in answer:

This can surely be more fine tuned, but it shows the idea. Adding
\renewcommand*{\mtcgapbeforeheads}{-50pt}
\renewcommand*{\mtcgapafterheads}{-40pt}
will the appearance adapt more to the one from question. The values are by the way the opposite of the defaults for these part toc commands.
BTW for users, who know what they are doing: scrwfile prints out a warning into the log file claiming to be in Alpha state. This warning can be removed with the package silence.
komabug.texin thedocsubtree for KOMA-Script. As alternative you could take a look onminitoc, loosely related to this is KOMA-Script (scrbook): onecolumn table of contents & minitoc in twocolumn document. – Speravir Jan 20 '14 at 22:57\@writefile) – Matthias Pospiech Jan 21 '14 at 08:13