LaTeX Forum

\documentclass[
  a4paper,
  12pt,
  BCOR=8mm,
  DIV=calc,
	pagesize=auto,
  headsepline,
	english,
  ngerman,
	pageanchor=false,
  openany,
	DIV=12,
	]{scrbook}


\usepackage[english,ngerman]{babel}
\usepackage{setspace}
\usepackage[section]{placeins}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{textcomp}
\usepackage{amsfonts,amsmath,amsthm}
\usepackage{dsfont}
\usepackage{subfig}
\usepackage{nicefrac}
\usepackage[toc]{appendix}
\usepackage{multirow}
\usepackage{todonotes}
\usepackage{courier}
\usepackage[utf8]{inputenc}
\usepackage[Sonny]{fncychap}
\usepackage[hang,small]{caption}
\usepackage{longtable}
\usepackage{blindtext}
\usepackage{scrlayer-scrpage}
\usepackage{hyperref}


\begin{document}
% hier werden die einzelnen Kapitel jetzt mit \include eingebunden

\chapter{Diese Kapitelüberschrift ist ganz schön lang. Und zwar so lang, dass sie zwei Zeilen im Header benötigt, was anscheinend die Warnmeldung verursacht.}
\blindtext[10]

\end{document}

Package scrlayer-scrpage Warning: \headheight to low.
(scrlayer-scrpage)                At least 28.99998pt needed,
(scrlayer-scrpage)                but only 18.125pt found.
(scrlayer-scrpage)                I'll enlarge \headheight, for further
(scrlayer-scrpage)                processing, but you should do this yourself,
(scrlayer-scrpage)                e.g., setting typearea's option
(scrlayer-scrpage)                `headheight=28.99998pt'.
(scrlayer-scrpage)                I'll also decrease \topmargin on input line 44

\documentclass[
  headheight=30pt,

\addtokomafont{caption}{\small}