I can confirm the issue is solved.

AFAICT it can be closed (I don't think I can close the issue myself)

Thanks again :)

The following mwe fails compilation with make4ht:
```latex
\documentclass{article}
\begin{document}
\begin{figure}\centering
\caption{This is the caption\\\hspace{\textwidth}This is the second line}
\end{figure}
\end{document}
```

The above is almost the same as this: https://tex.stackexchange.com/a/101624/56076

The only difference is the absence of the optional `[caption]` argument in the `\caption` macro.