You moved between networks, which caused your hostname to change
[1].
This meant that the xauth cookie for your current session no longer
matched, because it embeds the hostname, because the spec says that's
what you do.
We set up X with two levels of authentication. The second is the
traditional xauth cookie mechanism, but the first matches against UID
instead [2]:
% DISPLAY=:0 xhost
access control enabled, only authorized clients can connect
SI:localuser:ajax
So this keeps normal users from being affected when the hostname
changes. But once you've switched UID you fall back to the xauth cookie
and things break.
I wrote a patch for this once:
https://bugzilla.redhat.com/show_bug.cgi?id=679486#c43
And it got no conclusive testing or whatever, I forget. The patch itself
is in package git, although not enabled in the libXau build:
http://pkgs.fedoraproject.org/gitweb/?p=libXau.git;a=commitdiff;h=bdfc287...
If someone wanted to test it, that'd rule.