I'm really weirded out. The following letter compiles nicely:
\documentclass{scrlttr2}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{ngerman}
\usepackage[a4paper, textwidth=490pt, textheight = 690pt]{geometry}
\setkomavar{signature}{John Doe}
\begin{document}
\begin{letter}{Dr X \\ Evil Corp}
\setkomavar{subject}{Application}
\opening{Dear Dr X,}
blabla
\closing{Kind regards,}
\end{letter}
\end{document}
But when I include an (existing) signature image by replacing
\setkomavar{signature}{John Doe}
with
\setkomavar{signature}{\includegraphics{sig} \\\\ John Doe},
my compiler refuses to create a pdf, saying
Undefined control sequence \closing{Kind regards,}
I don't know what to do.
graphicx
), everything compiles fine.