I've just pushed the commit with changes to the repo. Hope it doesn't break anything.

i don't see a problem with making everything \long in our case.

but why did it start to fail at this late date?
because of the lineno stuff? i don't see that changing pend:def.

I wonder if it could be as simple as a missing \long. I hope Michal can look into it (and your other report(s)). Michal? Sadly, I could not locate the right place without more research.

The last change noted in the tex4ht ChangeLog for these classes was 2015-02-07. I don't know what might have changed recently to affect it. Sigh.

More information also here

http://tex.stackexchange.com/questions/319661/tex4ht-fails-to-compile-footnote-when-there-is-blank-line-with-scrartcl-class

This MWE fails to compile with tex4ht because there is blank line in footnote

\documentclass[11pt]{scrartcl}
%\documentclass[11pt]{article}
\IfFileExists{luatex85.sty}{\usepackage{luatex85}}{}
\begin{document}

Then \footnote{Another way to find particular

solution is by gussing.},

\end{document}

The file compiles OK with lualatex and pdflatex. Changing the class to article instead of scrartcl the error goes away.

TL 2016 on Linux mint.