X “Can’t open display: :0” while DISPLAY variable is correct

I’m not able to start any GUI applications as a root user: # pgrep -lf Xorg 1590 /usr/bin/Xorg -br -nolisten tcp :0 vt7 -auth /var/lib/xdm/authdir/authfiles/A:0-PNnJzp # echo $DISPLAY :0 # xeyes No protocol specified Error: Can't open display: :0 # firefox No protocol specified No protocol specified Error: cannot open display: :0 # xcalc No … Read more