No such method 'RegisterClient'

[dino99]

After validating the greeter password to open the gnome-shell session, the system takes too much time to find/load the session.
And this is logged into x-0greeter.log:

(unity-settings-daemon:1570): WARNING **: Unable to register client: GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod: No such method 'RegisterClient'

.... explaining it cant find as quickly as it should

later, it also logged:
(unity-settings-daemon:1570): dconf-WARNING **: failed to commit changes to dconf: The connection is closed

** (unity-settings-daemon:1570): WARNING **: Lost or failed to acquire name org.gnome.Mutter.IdleMonitor

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: unity-settings-daemon 15.04.1+16.04.20160121-0ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-1.15-generic 4.4.0
Uname: Linux 4.4.0-1-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.19.4-0ubuntu1
Architecture: amd64
CurrentDesktop: GNOME
Date: Wed Jan 27 17:19:16 2016
SourcePackage: unity-settings-daemon
UpgradeStatus: No upgrade log present (probably fresh install)

[dino99 (9d9)]

lightdm.log shows some confusing log:

Seat seat0: Display server ready, starting session authentication
[+20.61s] DEBUG: Session pid=1375: Started with service 'lightdm-greeter', username 'lightdm'
[+20.67s] DEBUG: Session pid=1375: Authentication complete with return value 0: Success

lightdm-greeter is now named lightdm-gtk-greeter into the xenial archive (not installed) but unity-greeter is used

note: a gnome-shell session should used its own greeter, a unity session its own too; but mixing with not updated package name is building a mess.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Max (maxmax)]

Ubuntu 16.04.3
unity-settings-daemon 15.04.1+16.04.20160701-0ubuntu1
unity-greeter 16.04.2-0ubuntu1
lightdm 1.18.3-0ubuntu1.1
xorg 1:7.7+13ubuntu3

/var/log/lightdm/seat0-greeter.log
** (unity-settings-daemon:1692): WARNING **: Unable to register client: GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod: No such method 'RegisterClient'
[+18.38s] DEBUG: menubar.vala:537: Adding indicator object 0x9d63b0c at position 3
[+29.15s] DEBUG: Providing response to display manager
[+29.15s] DEBUG: Wrote 24 bytes to daemon
[+29.22s] DEBUG: Read 8 bytes from daemon
[+29.22s] DEBUG: Read 13 bytes from daemon
[+29.22s] DEBUG: Authentication complete for user p with return code 0
[+29.22s] DEBUG: Starting session ubuntu
[+29.22s] DEBUG: Wrote 18 bytes to daemon
[+29.22s] DEBUG: Read 8 bytes from daemon
[+29.22s] DEBUG: Read 4 bytes from daemon
[+29.24s] DEBUG: unity-greeter.vala:611: Got a SIGTERM
[+29.24s] DEBUG: settings-daemon.vala:78: Failed to acquire name org.gnome.SessionManager
[+29.24s] DEBUG: settings-daemon.vala:105: Failed to acquire name org.gnome.ScreenSaver
[+29.24s] DEBUG: unity-greeter.vala:135: Failed to acquire name com.canonical.Unity
[+29.24s] DEBUG: unity-greeter.vala:619: Cleaning up
[+29.24s] DEBUG: unity-greeter.vala:627: Upstart exited with return value 0
[+29.24s] DEBUG: unity-greeter.vala:639: AT-SPI exited with return value 0
[+29.24s] DEBUG: unity-greeter.vala:645: Exiting