Here is the solution. It could be useful for anyone else.
After uploading .xml config files via ssh (as root), ownerships and permissions must be adjusted.
chown lsadm:lsadm /opt/lsws/conf/*.xml
chmod 600 /opt/lsws/conf/*.xml
chown lsadm:lsadm /opt/lsws/conf
chmod 700 /opt/lsws/conf
killall -9 lshttpd
That's all. No session errors.