It seems this might have happened with the upgrade to 9596. When I try to add a Linux based monitor it gets denied on the destination host. The user account works fine, and existing monitors seem to be okay. However, when I try to add a new monitor it fails and I get the following log in /var/log/secure. This error is what you get when a blank username is sent to SSH.
Jul 3 15:35:43 mail01 sshd[27043]: Invalid user from 10.10.10.10
Jul 3 15:35:43 mail01 sshd[27043]: input_userauth_request: invalid user [preauth]
Jul 3 15:35:43 mail01 sshd[27043]: Failed none for invalid user from 10.10.10.10 port 34608 ssh2
Jul 3 15:35:43 mail01 sshd[27043]: Received disconnect from 10.10.10.10: 11: [preauth]
Thanks,
-John