PUTTY X11 Forwarding does not work: no DISPLAY variable set

I want to get a GUI to an ubuntu server. Locally, I installed XMING & PUTTY and enabled the X11 forwarding. I also tried to specifically set the display location to localhost:0, because I read that it could be helpful. Unfortunately, after watching multiple youtube tutorials and reading through dozens of guides, I simply cannot get it to work. Whenever I connect to the server, the DISPLAY variable is not initialized. I did the following: - enable X11Forwarding in /etc/ssh/ssh_config and /etc/ssh/sshd_config - install xauth on the server - try manually setting the DISPLAY variable to :0.0, =:0, =:1 etc. Does anyone know how to fix or debug this properly? EDIT: I **might** know the issue: I can only login to the server as root account! Could that maybe cause the problem? ----------