I followed this guide to setup my Odoo Community server: https://www.linode.com/docs/websites/cms/install-odoo-10-on-ubuntu-16-04

(See in particular the section "Create an Odoo Service")

My Odoo installation works absolutely fine with just the default addons path.

My Odoo installation is in /opt/odoo/odoo10.

The relevant part of my service file reads:

[Service]
Type=simple PermissionsStartOnly=true SyslogIdentifier=odoo-server User=odoo Group=odoo 
ExecStart=/opt/odoo/odoo10/odoo-bin --config=/etc/odoo-server.conf --addons-path="/opt/odoo/odoo10/addons/,/opt/odoo/odoo10/custom-addons/"
WorkingDirectory=/opt/odoo/odoo10/

In my server.conf file, I also have the line:

addons_path = /opt/odoo/odoo10/addons, /opt/odoo/odoo10/custom-addons

When I attempt to start the Odoo server, I get the error:

odoo-server[1303]: odoo-bin: error: option --addons-path: The addons-path '/opt/odoo/odoo10/custom-addons/' does not seem to a be a valid Addons Directory!

I've double checked the permissions on the directory, and the odoo system user is the owner, with proper chmod permissions.

Please advise.