The following example won't compile (pdflatex). You can test that it compiles WITHOUT the package ngerman. But the the quotation marks are not correct. I don't know anything about the interior of packages, but the interference of these two packages seems very strange to me. Here is my example:
\documentclass[11pt, a4paper, german]{scrartcl}%Koma-scriprt!
\usepackage{scrlayer-scrpage}
\usepackage[utf8]{inputenc}
\usepackage{ngerman}
\usepackage{dcolumn}
\begin{document}
Nun probieren wir die "`Anführungszeichen"' hier.
%\end{document}
\begin{tabular}{D{,}{,}{-1}p{2 cm}D{,}{,}{-1}p{2 cm}D{,}{,}{-1}}
31,5 & & 42,39 & & 312,1\
9,123 & & 17,2 & & 99,53\
58,9 & & 210,15 & & 201,4\
64,27& & 29,314 & & 32,39\
50,41 & & 151,3 & & 26,1\
\hline
154,203 & & 410,354 & & 671,52
\end{tabular}
\end{document}
ngermanis unsupported and was last updated in 1998. It's been superseded by thebabelpackage -- with language optionngerman-- for a decade or more. Happily, your code compiles just fine if I replace\usepackage{ngerman}with\usepackage[ngerman]{babel}. – Mico Mar 20 '23 at 23:34