Solved, I was now able to run the conversion via the command line like this:
C:Program FilesPDF24>"C:Program FilesPDF24jrebinjava.exe" -cp "C:Program FilesPDF24libjar*" -Dwindows.acp=1252 -XX:MaxRAMPercentage=80 "org.pdf24.Converter" "-dpi=300" "-imageQuality=85" "C:temptest.pdf" "C:temptest.svg"

(as learned via https://help.pdf24.org/de/fragen/frage/qualitaetsunterschied-toolbox-doctool/)