lightdm does not boot into X | Plymouth error

[UH]

It's an old problem i have since when lxdm was abbanded in favour of
lightdm (but i didn't care hoping the next dist upgrade would
resolve). Now, unfortunately the problem remains.

While lxdm boots flawlessly, lightdm produces the following error:

------------
mountall: Plymout Befehl fehlgeschlagen
mountall: Keine Verbindung zu Plymouth
------------
(I translate:
Plymouth command could not be executed
No connection to Plymouth)

Now, i googled for this error, found a lot of thread and proposed
solution but nothing helped in my case. Especially the noplymouth boot
option did not have any effect, de-installation of Plymouth seemed to
me a to big risk to destroy more than to save.

So, i looked in the /var/log/lightdm/lightdm.log and found something
that might be a key for a solution, but i don't have any idea HOW . .
.

Extraction from lightdm.log:
--------------
[+0.71s] DEBUG: Got signal from X server :0
[+0.71s] DEBUG: Stopping Plymouth, X server is ready
[+0.73s] DEBUG: Connecting to XServer :0
[+0.75s] DEBUG: Starting greeter
[+0.75s] DEBUG: Started session 1257 with service 'lightdm-greeter',
username 'lightdm'
[+1.67s] DEBUG: Session 1257 authentication complete with return value
0: Success
[+1.67s] DEBUG: Greeter authorized
[+1.67s] DEBUG: Logging to /var/log/lightdm/x-0-greeter.log
[+1.71s] DEBUG: Session 1257 running command
/usr/lib/lightdm/lightdm-greeter-session /usr/sbin/lightdm-gtk-greeter
[+2.34s] DEBUG: Greeter closed communication channel
[+2.34s] DEBUG: Session 1257 exited with return value 1
[+2.34s] DEBUG: Greeter quit
[+2.34s] DEBUG: Failed to start greeter
[+2.34s] DEBUG: Stopping display
[+2.34s] DEBUG: Sending signal 15 to process 1245
[+3.31s] DEBUG: Process 1245 exited with return value 0
[+3.31s] DEBUG: X server stopped
[+3.31s] DEBUG: Removing X server authority /var/run/lightdm/root/:0
[+3.31s] DEBUG: Releasing VT 7
[+3.31s] DEBUG: Display server stopped
[+3.31s] DEBUG: Display stopped
[+3.31s] DEBUG: Stopping X local seat, failed to start a display
[+3.31s] DEBUG: Stopping seat
[+3.31s] DEBUG: Seat stopped
[+3.31s] DEBUG: Required seat has stopped
[+3.31s] DEBUG: Stopping display manager
[+3.31s] DEBUG: Display manager stopped
[+3.31s] DEBUG: Stopping daemon
[+3.32s] DEBUG: Exiting with return value 1
----------------

Anyone out there with an idea? TIA