Never mind, I found out by myself. The user rights weren't the real problem. The real problem was that the various tutorials and postings regarding Ubooquity on Synology NAS never mention that first time the server is up you have to go to <NAS-IP>:2202/admin, not to the server address alone.

To use root you have to connect with ssh as admin, then run the command "sudo -i" and enter the admin password again. You'll be root (cf. https://www.synology.com/en-us/knowledgebase/DSM/tutorial/General/How_to_login_to_DSM_with_root_permission_via_SSH_Telnet).