In the documentation it says:
To authenticate an additional backup server you have to ask the server administrator for the server key (It is shown on the status page)
I thought I have seen it before in the GUI of the server, but now I cannot find it.
Sorry, no idea.
Have you tried the second option?
Another option is to use the same key and private/public key pair for all backup servers (The private/public key pair is present starting from UrBackup Server 1.4 as urbackup/server_ident.priv and urbackup/server_ident.pub).