ssh -X fail with Error: Can't open display:

[powah]

I can ssh -X to a RHEL4 linux and a HP-UX B.11.23 U ia64 computer from a FC6 linux computer.
I can run
xclock or xterm
on the RHEL4 linux computer.
However, running
xclock or xterm
on the HP-UX B.11.23 U ia64 computer has these errors:
bash-3.2# xclock
Error: Can't open display:
Error: Couldn't find per display information

bash-3.2# xterm
xterm Xt error: Can't open display: %s

bash-3.2# sshd -version
sshd: illegal option -- v
OpenSSH_4.5p1+sftpfilecontrol-v1.1-hpn12v14, OpenSSL 0.9.7l 28 Sep 2006
HP-UX Secure Shell-A.04.50.021, HP-UX Secure Shell version

Using "ssh -Y" to login to the RHEL4 and HPUX computer get the same result.

[Nokia]

Use

ssh -vY

to see what's wrong. Increase verbosity by adding v's

[powah]

Using "ssh -vX"
...
debug1: Remote: Can't get IP address for X11 DISPLAY.
...

[Nokia]

DNS issue ? Post full output please.