documentclass[12pt]{article}
usepackage[utf8]{inputenc}
usepackage{trackchanges}
begin{document}
    text remove{öäü}
end{document}

! Argument of UTFviii@two@octets has an extra }.
<inserted text>
par
l.6 text remove{öäü}
I've run across a `}' that doesn't seem to match anything.

documentclass[12pt]{article}
usepackage[utf8]{inputenc}
usepackage{soulutf8}
usepackage{trackchanges}
begin{document}
    text remove{öäü}
end{document}

documentclass[12pt]{article}
usepackage[utf8]{inputenc}
usepackage{trackchanges}
renewcommand{stcolor}[2]{textcolor{#1}{#2}}
begin{document}
    text remove{öäü}
end{document}