Some things to try:
check X11Forwarding is enabled in sshd_config
try 'ssh -X localhost' on the problem machine and see if you can open windows in the ssh session locally (it will still be using X11Forwarding through ssh)
Try 'ssh -Y' instead of 'ssh -X' to enable trusted X11Forwarding
Try logging in as a different user, try root if it's allowed.
Restart the problem machine.
reinstall openssh server on the problem machine (yum remove openssh, yum install openssh)