When using the user name and password, make sure that the username and password entered on the source instance in Staging -> Servers for the target server (it can be dummy username and pwd, does not have to be real user account) is matching the values entered in the target instance in Settings -> Versioning & Synchronization -> Staging. Sample screen shot (green = source, red = target). And also, marked orange in the screen shot make sure the site level settings are set or inherited from the global settings.
When using X.509 certificate, you need to install the cert correctly. Please see the documentation.