I changed distro (centos 5.1) not OS and here it works fine! Try yourself.

2008/5/27 Sebastien Bacher <seb128@ubuntu.com>:
you should rather switch to an another os since the bug is a GNOME one
and not an ubuntu one and other distribution will likely the a similar
issue

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Remote Login via XDMCP is not working
https://bugs.launchpad.net/bugs/150193
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