Assuming you're using certificates to ssh into your server, I'd bet your problem is related to the fact that /etc/rc.local is sourced as root when launched from the init sequence.
If this is it you have two alternatives to work it out:
1) Generate a certificate for the user root on your local computer and ssh-copy-id it to the authorized keys on server@server. This way your remote directory will be mounted locally as root.
2) Modify your rc.local script to run the sshfs command as your herbert user, that is:
Doing it this way, your remote directory will be mounted as herbert locally.
sudo herbert -c "sshfs -o idmap=user server@server: /home/herbert/Server"