Imagine I login into xdm, then run screen,
then detach screen, quit X, login into xdm again,
then reattach screen.

With earlier versions I could then run from shells in this screen
any X application (maybe changing DISPLAY if I login into different X server)

Now in this case I receive:
$ xlsfonts                                                                  
Xlib: connection to ":6.0" refused by server
Xlib: Invalid XDM-AUTHORIZATION-1 key (failed key comparison)
xlsfonts:  unable to open display ':6.0'

(I logged in on the same Xserver :6).

It is NetBSD-current 3.99.3 and XFree86 4.4.99.22

Why can it be so?  What changed?  Can it be PAM related?