Adding chapter thumbs to an appendix with KOMA-Script

Question

Including an appendix chapter using \include{chapter_appendixA} results in a 'weird error' in overleaf (compiling with XeLaTeX):

! Missing number, treated as zero.

                 p

l.1 \chapter{chapter_appendixA} A number should have been here; I inserted `0'.

! Package PGF Math Error: Unknown function `A' (in '-A*\LabelSize '). See the PGF Math package documentation for explanation. Type H for immediate help.

The chapter_appendixA.tex file only includes the following:

\chapter{Appendix}
Some text

Other appendix examples do not include a number there. The error arises when activating the gray chapter label boxes using \AddLabels from "Show current chapter number on each page margin" Does anyone have an idea?

My main code is the following:

\documentclass[10pt,twoside,parskip=half,table,xcdraw]{scrbook}
\usepackage[
  paperwidth=170mm,
  paperheight=240mm,
  top=2.5cm,
  bottom=3cm,
  inner=30mm,
  outer=25mm,
  heightrounded=true
]{geometry}

\input{preamble.tex}

\raggedbottom

% avoid annoying errors when using Koma-Script with bibtex
\DeclareOldFontCommand{\bf}{\normalfont\bfseries}{\mathbf}


\begin{document}

%%%%%%%%%%%%%%%%%%
%% FRONT MATTER %%
%%%%%%%%%%%%%%%%%%

\frontmatter
\tableofcontents

%%%%%%%%%%%%%%%%%
%% MAIN MATTER %%
%%%%%%%%%%%%%%%%%

\mainmatter
\rehead{\small{Chapter \thechapter}}

\chapter{A chapter}
    Some text
\AddLabels  

%%%%%%%%%%%%%%
%% APPENDIX %%
%%%%%%%%%%%%%%

\appendix
\include{chapter_appendixA}

\end{document}

With accompanyingpreamble.tex:

%%%%%%%%%%%
%% FONTS %%
%%%%%%%%%%%
\usepackage{xcolor}
\usepackage{libertine}
\setmainfont{Linux Libertine O} 
\setsansfont{Linux Biolinum O}

%Set chapter titles and numbering
\usepackage[headsepline=true]{scrlayer-scrpage}
\clearpairofpagestyles


%%%%%%%%%%%%%%%%%%%%%%%
%% chapter numbering %%
%%%%%%%%%%%%%%%%%%%%%%%

%\renewcommand*{\thesection}{\thechapter.\arabic{section}}
\renewcommand{\sectionmark}[1]{\markright{#1}{}}

% Create header for odd (o) and even (e) pages
\lehead{\thepage}\rohead{\thepage}
\lohead{{\small\rightmark}}
\setkomafont{pagehead}{\normalfont\normalcolor}

\usepackage[times]{quotchap}   % fancy chapter beginning

\pretolerance=10000
\tolerance=10000

% Gray boxes for chapter numbering on pages
\usepackage[contents={},opacity=1,scale=1,color=white]{background}
\usepackage{tikzpagenodes}
\usepackage{totcount}
\usetikzlibrary{calc}

\newif\ifMaterial

\newlength\LabelSize
\setlength\LabelSize{2.5cm}
\AtBeginDocument{%
\regtotcounter{chapter}
%\setlength\LabelSize{\dimexpr\textheight/\totvalue{chapter}\relax}
\ifdim\LabelSize>2.5cm\relax
  \global\setlength\LabelSize{2.5cm}
\fi}

\newcommand\AddLabels{%
    \Materialtrue%
    \AddEverypageHook{%
        \ifMaterial%
        \ifodd\value{page} %
        \backgroundsetup{
          angle=90, position={current page.east|-current page text area.north  east}, vshift=8pt, hshift=-\thechapter*\LabelSize,
          contents={%
          \tikz\node[fill=gray!30,anchor=west,text width=\LabelSize,
            align=center,text height=15pt,text depth=10pt,font=\large\sffamily] {\thechapter};  }%
        }
         \else
         \backgroundsetup{
          angle=90, position={current page.west|-current page text area.north west}, vshift=-8pt, hshift=-\thechapter*\LabelSize,
          contents={%
          \tikz\node[fill=gray!30,anchor=west,text width=\LabelSize,
            align=center,text height=15pt,text depth=10pt,font=\large\sffamily] {\thechapter}; }%
         }
     \fi
     \BgMaterial%
     \else\relax\fi}%
}

\newcommand\RemoveLabels{\Materialfalse}
\usepackage{afterpage}
\newcommand\RemoveThisLabel{%
    \Materialfalse
    \afterpage{\global\Materialtrue}%
}

Answer 1

KOMA-Script has a more robust approach to adding chapter thumbs using its scrlayer-scrpage package. Download the examples from the KOMA-Script book, and use the files in the Anhang-B folder. Here is a translated version of the example with an appendix added, which will work if the chapterthumb.sty file from the examples is in the same folder:

\documentclass{scrbook}
\usepackage{chapterthumb}
\usepackage{scrlayer-scrpage}
\AddLayersToPageStyle{@everystyle@}{chapterthumb}
\addtokomafont{chapterthumb}{\bfseries}

\begin{document}
\chapter{In the beginning it starts}\dots
\chapter{continuing}\dots
\chapter{further}\dots
\chapter{and even further}\dots
\chapter{And always on}\dots
\chapter{Goes the journey}\dots
\chapter{out}\dots
\chapter{And further out}\dots
\chapter{Always further out}\dots

\appendix

\chapter{Until the end}\dots

\end{document}

If you wish to customize it, this has been addressed several times before; see the chapterthumb tag.