Gtk-warning: cannot open display

[Johan N-P <johan_np post netlink se>]

Hi all,

I have a really weird problem. Until yesterday my gtk program worked
fine, but suddenly it stopped working. Now it  just tells me
"gtk-warning: cannot open display" when I try to run it.
$DISPLAY is :0.0 as it should be. And yes, I'm running it as the user
who started X.

When I compile and run other gtk  programs they work fine, it's just my
program that behaves like this. I stepped through it in gdb to see where
it happens, and I found that it's always gtk_init that gives this error
without any apparent reason (there is absolutely nothing wrong with argc
and argv or the way I supply them).

What could tbe the reason for this?

Johan