tabu uses wrong box with longtable 4.14 and newer.

[u-fischer]

longtable has changed the internal box names, instead of \@ne is now uses \LT@gbox. tabu must be adapted accordingly

see also https://tex.stackexchange.com/questions/600724/dimension-too-large-after-recent-longtable-update

\documentclass{article}
\usepackage{longtable}
\usepackage{tabu}%
\makeatletter
\def\tabuendlongtrial{% 
    \LT@echunk  \global\setbox\LT@gbox \hbox{\unhbox\LT@gbox}\kern\wd\LT@gbox
                \LT@get@widths
}% 

\makeatother

\begin{document}
\begin{longtabu}  {llX}
a & b & c\\
\end{longtabu}
\end{document}

[Lenchik]

Is there a solution with if/fi for this patch being in a preamble of document being comaptible with older compilers (like the one from TexLive 2018)? Just to have document compilable without errors with old and new compilers.

[FrankMittelbach]

@Lenchik this issue has only been opened in case there is ever going to be a new maintainer for the package. The package is badly broken and I rather doubt that this will happen, so the only advice I can give you is not to use it. It is not as if it now has an issue, it had issues for a long time and they are just increasing. In my opinion it should be taken off the distributions.

[u-fischer]

@Lenchik you can test for the date of a package with \@ifpackagelater. But as Frank wrote: much better is not to use tabu at all.

[TC-RAC]

Is there any good guidance anywhere of what to do if working with old documents that use tabu and how to update them to something that does work?

[u-fischer]

@TC-RAC remove tabu and rework the tabulars. How depends on their layout. Often tabu isn't really needed and you can use tabular, in other cases try tabularray.