I noticed that adding pdfx package to my document stopped the soul package from highlighting the text in yellow.
\documentclass[a4paper,12pt]{article}
\usepackage[a-1b]{pdfx} % comment this out
\usepackage{color}
\usepackage{soulutf8}
\begin{document}
\hl{TODO}
\end{document}
The example above does not report any errors, but results in non-highlighted text, as shown below. Removing the pdfx package reverts everything to normal behavior.
The order of the packages does not seem to have an immediate effect.

pdflatex(TeXLive 2018). To help you, we need to know the exact error message or thelogfile. – Marian G. Dec 13 '19 at 10:57pdflatex -synctex=1 -shell-escape -interaction=nonstopmode %.texTeXLive version:
– jureslak Dec 13 '19 at 11:03texlive-bin 2019.51075-4