documentclass[]{article}
usepackage{luacode}

newcommand{intString}[1]{luaexec{tex.print("#1")}}

begin{document}
section{intString{test}} %this breaks
intString{lofCaption}  % this works

cleardoublepage
addcontentsline{toc}{chapter}{intString{lofCaption}} % this breaks
protectlistoffigures

cleardoublepage
addcontentsline{toc}{chapter}{intString{lotCaption}} % this breaks
protectlistoftables
end{document}

@writefile{toc}{contentsline {section}{numberline {1}begingroup escapechar 92 newlinechar 10 edef \{\}edef nobreakspace  {}{~}let %=%let #=#endgroup test}{1}}