think there is nothing more to do here, closing.

Quick update. I think we are getting closer.

I found the line which causes it to fail. The \begin{align}...\end{align} has in inside it now 39 equations, each now on one line. This is all in file foo_v2.tex which is in same folder https://www.12000.org/tmp/051020/

When I commented line 39 (last equation) inside the align environment, then it compiles OK.

Uncommenting line 39 the error comes back.

So I thought may be tex4ht needs more resources? So edited

/usr/local/texlive/2020/texmf.cnf and added these

buf_size=90000000
pool_size=9000000
main_memory=8000000

Then run

sudo /usr/local/texlive/2020/bin/x86_64-linux/fmtutil-sys --all

But the problem is still there. So it is not resource problem. The problem is in the

/usr/local/texlive/2020/texmf-dist/tex/latex/base/ts1cmr.fd

And only shows when running tex4ht in mathjax mode.

I see nothing special about last equation in the align environment, so I do not know why removing it removes the error.

I have one \begin{align}...\end{align} which contains few equations.

This is auto-generated from Mathematica. Now the only thing, is that there is no new line between the equations. So it looks like this

\begin{align} .....\\ &=.. \\ ... &= ....\\ .. \end{align}

All on one LONG line.

This causes no problem at all to lualatex. It compiles OK and the output is correct.

But when I do

make4ht -ulm default -a debug foo.tex "htm,mathjax"

I get

----------------------
(/usr/local/texlive/2020/texmf-dist/tex/generic/tex4ht/html5.4ht)) (./foo.aux)
(/usr/local/texlive/2020/texmf-dist/tex/latex/base/ts1cmr.fd)
! Bad register code (65536).
\__regex_query_set:nnn ... \l__regex_curr_pos_int
{#1}\int_incr:N \l__regex_...

l.8 ...4 \left(7+2 \sqrt{14}\right)}}\\ \end{align*}

?
----------------------

Since the file is long, I can not put it here (actually the file is only few lines, but the line is over 110,000 characters long. It looks like

==========================
\documentclass[11pt]{article}
\usepackage{amsmath}
\begin{document}
\begin{align} .....\\ \end{align}
\end{document}
==================================

So I put foo.tex and also foo.pdf (generated from lualatex) in this folder

https://www.12000.org/tmp/051020/

Also, when I tried to compile it using

>make4ht -ulm default -a debug foo.tex "htm"

i.e. no mathjax, it did not crash, but tex4ht generated hundreds of image .png files.

Question is, why does it crash like this? Is there a limit on how long a line can be in Latex for use with tex4ht/mathjax?

Using TL 2020.