I think that I've fixed this issue finally. We need to make the problematic commands robust. So for the future reference, instead of

\def\:temp#1{...}
\HLet\foo\:temp

it is better to use the following code for the commands that can be used in captions or sectioning commands:

\ProvideDocumentCommand\foo:temp{m}{...}
\HLet\foo\foo:temp

I think one would never want any \caption contents expanded when writing it to a file? Can't quite imagine when ...

Thank you, Michal. Will try the \protect thing. If that works, it may be worthwhile to try and add some logic to do that automatically. I'm actually using tex4ebook, but it uses tex4ht internally, and that's where I ran into this issue.

Recently when trying to convert a scientific paper to an eBook using tex4ht, I ran into some confusing errors. I tracked it down to a macro that used \left and \right inside of the caption of a labeled figure, which only seems to be a problem with hyperref included. A simple example to reproduce this:

When compiled with `mk4ht htlatex main2.tex`, this gives:

Interestingly, a small change to \thefigure results in a completely different error:
\documentclass{article} \usepackage{hyperref} \begin{document} \renewcommand{\thefigure}{S\arabic{figure}} \begin{figure} \caption{$\left(A\right)$} \label{a} \end{figure} \end{document}

Now gives:
(./main2.aux) [1] [2] ! You can't use `\relax' after \the. <recently read> \c@ l.10 \label{a} ?

This only seems to happen with the very specific combination of factors I have outlined above. Not including \left and \right, not having them inside a \caption, not having the \label, or not including \hyperref all seem to make this go away, and it compiles with regular LaTeX just fine. I'm afraid I don't know enough about the macros you're using to be able to debug this.