xdm failing to start on FBSD 10.0 r2340030 erratically

I loose hair ...
Since yesterady's "make world" (last make world: the day before
yesterday), getting FreeBSD 10.0-CURRENT/amd64 to r234000 or so, the X11
system on all of our FreeBSD 10.0-CUR/amd64 boxes start rejecting the
start of xdm display manager. xdm is started from /etc/ttys on ttyv7.
This worked before flawless.

At this very moment, I do have X11 started via xdm - but this is a
erratic and non-reproduceable process!

This morning, I update world and kernel to r234030. I recompiled many
ports via "portmaster -f xorg xdm", hoping the new kernel/world could
affect the ports, but this isn't.

Starting Xorg X11 server works fine. xdm fails. The log in
/var/log/xdm.log looks like:

Build Date: 07 April 2012  04:51:08PM

Current version of pixman: 0.24.2
        Before reporting problems, check http://wiki.x.org
        to make sure that you have the latest version.
Markers: (--) probed, (**) from config file, (==) default setting,
        (++) from command line, (!!) notice, (II) informational,
        (WW) warning, (EE) error, (NI) not implemented, (??) unknown.
(==) Log file: "/var/log/Xorg.0.log", Time: Sat Apr  7 18:38:24 2012
(==) Using config file: "/etc/X11/xorg.conf"
xdm info (pid 2055): sourcing /usr/local/share/X11/xdm/Xsetup_0
xdm error (pid 2050): Unknown session exit code 2560 from process 2055
xdm info (pid 2050): Exiting

When running properly, this occurs:

Build Date: 07 April 2012  04:51:08PM

Current version of pixman: 0.24.2
        Before reporting problems, check http://wiki.x.org
        to make sure that you have the latest version.
Markers: (--) probed, (**) from config file, (==) default setting,
        (++) from command line, (!!) notice, (II) informational,
        (WW) warning, (EE) error, (NI) not implemented, (??) unknown.
(==) Log file: "/var/log/Xorg.0.log", Time: Sun Apr  8 13:10:41 2012
(==) Using config file: "/etc/X11/xorg.conf"
xdm info (pid 18378): sourcing /usr/local/etc/X11/xdm/Xsetup_0
xdm info (pid 18378): sourcing /usr/local/etc/X11/xdm/GiveConsole
xdm info (pid 19848): executing session /usr/local/etc/X11/xdm/Xsession


Content of XSetup_0 is
#!/bin/sh
#xconsole -geometry 480x130-0-0 -daemon -notify -verbose -fn fixed
-exitOnFail
xsetroot -solid black


I feel a bit helpless around here since I can not get close to what is
happening. The erratic behaviour of starting xdm is frightening.
Starting xdm via /etc/ttys doesn't work at all, but sometimes, with a
bit luck, xdm starts when started from the console.

Regards,
Oliver