Here is a MWE:
\documentclass{kaobook}
\usepackage{mwe}
\begin{document}
\mainmatter
\begin{figure}
\begin{center} % culprit
\includegraphics[width=0.5\columnwidth]{example-image}
\caption{
A figure caption
}
\end{center} % culprit
\end{figure}
\begin{table}[h]
% \centering
\begin{tabular}{|c|c|c|c|}
\hline
H1 & H2 \tabularnewline
\hline
\hline
A & 1 \tabularnewline
\hline
B & 2 \tabularnewline
\hline
C & 3 \tabularnewline
\hline
D & 4 \tabularnewline
\hline
\end{tabular}
\caption{A table caption.}
\end{table}
\end{document}
Then pdflatex halts:
[1] (build/proposal-mwe-kao-2.mw) [2] (build/proposal-mwe-kao-2.mw)
(/usr/local/texlive/2021/texmf-dist/tex/latex/newpx/ot1zpltlf.fd)
(/usr/local/texlive/2021/texmf-dist/tex/latex/amsfonts/umsa.fd)
(/usr/local/texlive/2021/texmf-dist/tex/latex/microtype/mt-msa.cfg)
(/usr/local/texlive/2021/texmf-dist/tex/latex/amsfonts/umsb.fd)
(/usr/local/texlive/2021/texmf-dist/tex/latex/microtype/mt-msb.cfg)
(/usr/local/texlive/2021/texmf-dist/tex/latex/newpx/unpxmia.fd)
(/usr/local/texlive/2021/texmf-dist/tex/latex/newpx/unpxsym.fd)
(/usr/local/texlive/2021/texmf-dist/tex/latex/newpx/unpxsyc.fd)
! Package floatrow Error: Caption(s) lost.
See the floatrow package documentation for explanation.
Type H <return> for immediate help.
...
l.33 \caption{A table caption.}
?
If I remove the "culprit" lines, I get the output
I suppose the figure is centered anyway, but I'd like to understand what's the issue here. I have a document that I'm migrating from tufte-book to kaobook, and the center environment did not cause problems there.
Possibly related: How to solve the issue between side-by-side floats (tables) and floatrow package?
And if I'm interpreting this answer correctly, I should be using \centering instead of the center environment within a figure in any case.
Version Info:
$ pdflatex --version
pdfTeX 3.141592653-2.6-1.40.22 (TeX Live 2021)
kpathsea version 6.3.3
Copyright 2021 Han The Thanh (pdfTeX) et al.
There is NO warranty. Redistribution of this software is
covered by the terms of both the pdfTeX copyright and
the Lesser GNU General Public License.
For more information about these matters, see the file
named COPYING and the pdfTeX source.
Primary author of pdfTeX: Han The Thanh (pdfTeX) et al.
Compiled with libpng 1.6.37; using libpng 1.6.37
Compiled with zlib 1.2.11; using zlib 1.2.11
Compiled with xpdf version 4.03

! Undefined control sequence. \__hook begindocument/before ...hrm \theoremstyle(oh that's a different issue, I get the caption lost error in tl 2020) – David Carlisle May 28 '21 at 07:10