Created attachment 796417 [details]
lightdm_logs

When I start a session with lightdm(-gtk-greeter) all my graphical application
print the error/warning "no protocol specified". I searched for the issue a
long time and then I thought this is caused by an outdated configuration from
updating my openSUSE from one version to another several times. 
So reinstalling didn't work either.
Testing it with another user produced the same result.

Steps to reproduce:
1 .install lightdm with gtk-greeter
# zypper install lightdm lightdm-gtk-greeter
# update-alternatives --config default-displaymanager
-> set the value to lightdm-gtk-greeter

2. Restart your computer. (Restarting xdm.service didn't work to reproduce the
issue when another displaymanager/X Session was already active)

2. When the display manager shows up, switch to a session like icewm

3. login

4. start a terminal and type "firefox" for example. 

Then you should see "no protocol specified" several times. 
Personally I use i3 as my window manager.
When starting i3 from gdm everything looks fine. When I switch to lightdm after
gdm was used, the error also doesn't occur.
Starting a fresh X Session with lightdm causes this error.

/var/log/Xorg.0.log, .xsession-errors and /var/log/lightdm content attached.