lightdm - another login loop problem

Since last week I'm also faced with the endless-loop problem using ubuntu 14.04.3 (installed in dual-boot on HP 840 EliteBook).
Upon login the screen turns black and returns to the login interface.I have investigated a couple of things, but I am really stuck. Thus, I would appreciate, if someone could help me out. I have read most of the existing threads, but it seems that most problems were related to certain typos,
e.g.

The output of my .xsession-errors is:

/usr/sbin/lightdm-session: line 33: cat: command not found
/usr/sbin/lightdm-session: line 34: truncate: command not found
/usr/sbin/lightdm-session: line 106: ls: command not found
/usr/sbin/lightdm-session: line 117: exec: gnome-session not found

/etc/profile and ~/.bashrc seem to be working fine. When adding 'echo $PATH' to the end of each file, I get following output:

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games

But after logging into tty-console, the output of the $PATH environment variable is actually empty.

Moreover, I have tried:

• renaming/removing .Xauthority


• removing/re-installing/reconfiguring lightdm


• switching to gdm


• reinstalling xorg


• xorg


• adding a new user,

but the endless-loop effect remained. In addition, I've checked:

• permissions of tmp (drwxrwxrwt)


• /etc/environment (contains same path as $PATH above)


• /var/log/lightdm/lightdm.log (file is empty)


• .pam_environment (this file does not exist so nothing can be wrong here)

Since I am rather new to ubuntu, I would be grateful for some hints. Thanks in advance for your efforts!