Cannot log in to Unity after Lubuntu installation

[lucky2103]

Normally, I run Ubuntu 11.04

However, after installing the Lubuntu desktop environment side-by-side with the Unity desktop, I get a strange error message when trying to log in:

xsession: unsupported number of arguments (2); falling back to default session

After clicking 'Okay', it tries to log in, but all I get to see is the sky blue lubuntu screen.
It appears that the windows manager fails to load.

Any idea how to resolve this ?

[doogiekd]

this thread is working on a similar issue:

http://ubuntuforums.org/showthread.php?t=1846382