No hyphenation in TOC entries, please

Since you are using a KOMA class, the way to go is to use the tocstyle package:

The code:

\documentclass{scrreprt}
\usepackage[ngerman]{babel}
\usepackage[tocflat]{tocstyle}

\newtocstyle{raggedstyle}{%
  \settocstylefeature[0]{entryhook}{\bfseries}
  \settocstylefeature[0]{leaders}{\hfill}
  \settocfeature{raggedhook}{\raggedright}
  \settocfeature{spaceafternumber}{17pt}
}
\usetocstyle{raggedstyle}

\begin{document}

\tableofcontents

\chapter{Introduction}
\section{This kind of problem, that kind of problem, other ones and further considerations}
\section{More problems, even more problems, much worse than these, headline with no hyphenation and somo more text goes here for the example}

\end{document}

Refer to the package documentation for further adjustments.

I think I'd be inclined to use Gonzalo Medina's answer given you are using scrreprt (and skip loading tocloft), but a tocloft solution isn't too tricky (and also useful for memoir users):

\documentclass{scrreprt}
\usepackage[ngerman]{babel}
\usepackage{tocloft}
\cftsetindents{section}{0mm}{12mm}
\cftsetindents{chapter}{0mm}{12mm}

\makeatletter
% \renewcommand{\@tocrmarg}{2.55em plus1fil}
\renewcommand{\@tocrmarg}{\@pnumwidth plus1fil} % <-- Revised
\makeatother

\begin{document}
\tableofcontents
\chapter{Introduction}
\section{This kind of problem, that kind of problem, other ones and further considerations}
\section{More problems, even more problems, much worse than these, headline hyphenation, and blablablablablablablablabla and XYZ-blablablablablablablablabla}

\end{document}

You should, in general, make sure the revised value for \@tocrmarg (TOC Right MARGin) is greater than that for \@pnumwidth (Page NUMber WIDTH).

A solution with the titletoc package:

\documentclass{scrreprt}
\usepackage[showframe]{geometry}
%\usepackage[ngerman]{babel}
\usepackage{ragged2e}
\usepackage{fmtcount}
\usepackage{titletoc}
 \titlecontents{chapter}[12mm]
 {\contentsmargin{10mm}\bigskip\sffamily\bfseries\Large}
 {\contentslabel[\MakeUppercase{\romannumeral\thecontentslabel}]{12mm}}
 {}
 {\hfill\contentspage}[\medskip]


 \titlecontents{section}[12mm]
 { \rightskip=10mm plus 1fil\hyphenpenalty=10000\contentsmargin{2mm}}%
 {\contentslabel[\thecontentslabel.]{12mm}}
 {}
 {\hskip2pt\titlerule*[6pt]{.}\contentspage}


\begin{document}
\tableofcontents

\setcounter{page}{100}
\chapter{Introduction}
\section{This kind of problem, that kind of problem, other ones and further considerations}
\section{More problems, even more problems, much worse than these, headline hyphenation, and blablablablablablablablabla and XYZblablablablablablablablabla}

\end{document}