X-client can not get access to the display

[ivanov-void]

I'm trying to run the X-client xclock from a terminal. I'm typing

CTRL+ALT+F1

then

Mandriva Linux release 2011.0 (Official) for x86_64
Kernel 2.6.38.7-desktop-1mnb2 on a Dual-processor x86_64
localhost.localdomain login: ...
Password: ...

[root:localhost ~]# xclock -digital
Error: Can't open display:
[root:localhost ~]# DISPLAY=:0.0;export DISPLAY
[root:localhost ~]# xclock -digital
No protocol specified
Error: Can't open display: :0.0
[root:localhost ~]# export DISPLAY=localhost:11.0
[root:localhost ~]# xclock -digital
Error: Can't open display: localhost:11.0
[root:localhost ~]# startx --:11
xauth: file /root/.serverauth.10515 does not exist

Fatal server error:
Server already active for display 0
   If this server is no longer running, remove /tmp/.X0-lock
   and start again.

...

Invalid MIT-MAGIC-COOKIE-1 keyxinit: giving up
xinit: unable to connect to X server: Connection refused
xinit: server error
[root:localhost ~]#

In /etc/ssh/sshd_config i uncommented the following:

...
ListenAddress localhost
...
X11Forwarding yes
X11DisplayOffset 10
X11UseLocalhost yes
PermitTunnel yes
...

In reality, i want to run a Java-application written using Swing library, but when i enter

java -jar SpeedTest.jar

Get the same message, that X-client can not get access to the display.

What could be causing this?

I would be most grateful for the information!
Thank you!