[Oko]

On the server side

in sshd_config I set X11UseLocalHost "no"

That should be set to yes the way that I read which is default value for server side and X11 forwarding should be set to YES as well. Here is part of mine sshd_config which is not altered from the default installation since the only thing I do differently is disabling root login

#AllowAgentForwarding yes
#AllowTcpForwarding yes
#GatewayPorts no
#X11Forwarding no
#X11DisplayOffset 10
#X11UseLocalhost yes

Commented are default values. When you want something you have to
uncomment and put different value.

On the client side the default values are

# Host *
#   ForwardAgent no
#   ForwardX11 no
#   RhostsRSAAuthentication no
#   RSAAuthentication yes
#   PasswordAuthentication yes
#   HostbasedAuthentication no
#   GSSAPIAuthentication no
#   GSSAPIDelegateCredentials no
#   BatchMode no
#   CheckHostIP yes
#   AddressFamily any
#   ConnectTimeout 0

I think that ForwardX11 should be set to yes.

To be perfectly honest with you I have not used it for a while so I would have to look man pages and
few books to be able to give you precise answer.
You can do configuration per user so you do not have to alter default values in ssh_config system wide.