unable to forward X11 applications

Question

I installed a headless Debian server which has Wireshark installed. Now I would like to run Wireshark in a way that it uses X11 server running in my Linux Mint laptop. As a first step I permitted X11 forwarding in Debian SSH server configuration file:

root@A58:~# grep X11Forwarding /etc/ssh/sshd_config 
X11Forwarding yes
root@A58:~# 

Then I restarted SSH server with /etc/init.d/ssh restart and finally executed SSH client in my laptop using -X which enabled X11 forwarding. However, I still receive the "cannot open display" error:

root@A58:~# echo "$DISPLAY"
localhost:11.0
root@A58:~# wireshark 

(wireshark:3590): Gtk-WARNING **: cannot open display: localhost:11.0
root@A58:~# time xterm
Warning: This program is an suid-root program or is being run by the root user.
The full text of the error or warning message cannot be safely formatted
in this environment. You may get a more descriptive message by running the
program as a non-root user or by removing the suid bit on the executable.
xterm: Xt error: Can't open display: %s

real    1m3.131s
user    0m0.000s
sys 0m0.000s
root@A58:~#

Is the "$DISPLAY" variable correct? At the moment it's pointing to Debian server.. If the "$DISPLAY" variable is correct, then it's probably an authorization issue with X11 server?

X11 in laptop is started in following manner:

root      1113  0.1  4.3 101396 89992 tty7     Rs+   2013  44:28 /usr/bin/X :0 -nr -verbose -auth /var/run/gdm/auth-for-gdm-FHVC1j/database -nolisten tcp vt7