code produces error with lualatex but not with pdflatex. Why?

Question

I try to use tufte-book and can compile with pdflatex but not with lualatex (which I would prefer to use). The errors are

25: Argument of \MakeUppercase has an extra }. }{AF}
25: Paragraph ended before \MakeUppercase was complete. }{AF}
25: You can't use `\relax' after \the. }{AF}
25: Use of \__text_expand_space:w doesn't match its definition. }{AF}
25: Undefined control sequence. }{AF}
25: Missing number, treated as zero. }{AF}
25: Missing { inserted. }{AF}
25: Package soul Error: Reconstruction failed. }{AF}
25: Missing } inserted. }{AF}
28: Argument of \MakeUppercase has an extra }. \maketitle
28: Paragraph ended before \MakeUppercase was complete. \maketitle
28: You can't use `\relax' after \the. \maketitle
...

My attempts at checking give matching opening and closing braces, and I cannot find \MakeUppercase in the code.

Why does this code not work with lualatex? I have checked lualtex docs and a perhaps related question, but not found an answer. I use

This is LuaHBTeX, Version 1.15.0 (TeX Live 2022/Debian)  
(format=lualatex 2023.6.17)  27 JUN 2023 21:03
 restricted system commands enabled.

and think this similar question does not apply.

A MWE is:

\documentclass{tufte-book}
% Book metadata
\title{Ein Buch mit Titel}
% Prints an epigraph and speaker in sans serif, all-caps type.
\newcommand{\openepigraph}[2]{%
  %\sffamily\fontsize{14}{16}\selectfont
  \begin{fullwidth}
  \sffamily\large
  \begin{doublespace}
  \noindent\allcaps{#1}\% epigraph
  \noindent\allcaps{#2}% author
  \end{doublespace}
  \end{fullwidth}
}
\begin{document}
\frontmatter
\vfill
\openepigraph{%
Was soll man tun?
}{AF}
\vfill
\maketitle
% Start the main matter (normal chapters)
\mainmatter
Here the main matter
\end{document}