I had the same problem; I should have posted something to the list.
From memory, I had only installed the NetBSD 1.5 clients, having
previously installed the XFree86 4.0 servers (for ATI 128 support,
I believe).  My solution was to recompile and reinstall the
XFree86 4.0 distribution; not a great workaround, but it did
the job :-/

On 15 Jan 2001, Hauke Fath wrote:

> [Please keep me cc'ed as I am not subscribed]
> 
> Hi,
> 
> upgrading a NetBSD system always has a few surprises...
> 
> I moved a 1.4Q mac68k installation to 1.5, and as a result, the xdm setup
> refused any access to the local X server. It turned out that the default
> for "DisplayManager.DISPLAY.authName" has changed from "MIT-MAGIC-COOKIE-1"
> to "XDM-AUTHORIZATION-1".
> 
> Reverting to the former state fixed the setup. While it is not exactly
> relevant for my home setup that "XDM-AUTHORIZATION-1" is (said to be)
> safer: How would I get this to work? 
> 
> 	hauke
> 
> 

-- 
"Maybe this world is another planet's Hell." - Aldous Huxley