hmm I'm pretty sure its root with no password... you can edit /etc/shadow and remove the 2nd field on root and ubuntu and try again just in case
C:\Windows\System32>ssh -l root 192.168.178.184 root@192.168.178.184's password: Permission denied, please try again. root@192.168.178.184's password: Permission denied, please try again. root@192.168.178.184's password: root@192.168.178.184: Permission denied (publickey,password).