Try to edit /lib/systemd/system/serialoscd.service and replace
ExecStart=/usr/local/bin/serialoscd -c ~/.config/serialosc/
with
ExecStart=/usr/local/bin/serialoscd -c /root/.config/serialosc
(i.e.: replace ~ with /root and remove the trailing slash)
then do systemctl daemon-reload; systemctl restart serialoscd