Hello,
You might add 1 server in Directadmin interface, then check the file /usr/local/directadmin/data/admin/cluster.conf
for a syntax. Keeping in mind base64 is used for saved passwords.
And I would never add 20 servers in Directadmin multiserver. If you add all of them, you might face a significant degradation of speed when adding new users/domains, and editing DNS records in DirectAdmin interface.