XIO: fatal IO error 35 (Resource temporarily unavailable) on X server ":0.0"

Updating ports and source of my FreeBSD 9.0-CURRENT/amd64 host, equipted 
with a AMD/ATi HD4830 graphics board driven by X11 xf86-video-ati driver 
(which has been recently update as far as I saw),
resulted this morning in an 'un-login-able' box.

I see the xdm-login requester, but after successfully login, I see for a 
second the desktop (windowmaker), but X11 immediately dies and resets to 
the xdm requester again.

Login from another box and examining the ~/.xsession-errors shows only 
this entry:

---
foo.bar.org being added to access control list
XIO:  fatal IO error 35 (Resource temporarily unavailable) on X server 
":0.0"
       after 29 requests (29 known processed) with 0 events remaining.
wmaker warning: got signal 15 - exiting...
---

Recompiling xdm and xorg-server ( I did this desperately in the first 
place) didn't help very much.

I do not dare to update all the other boxes in my lab, since I suspect a 
similar problem since they have all the same or similar hardware (AMD 
graphics hardware, running X11, running FreeBSD 9.0-CURRENT/amd64).

What to do? What changed?

Regards,
Oliver