Can't XDMCP after server upgrade

[Val]

Hello.

I have run into an Xorg issue which unfortunately exhausted my own
knowledge very quickly, so I hope someone on this list might be able to
help.

I have a very old client workstation connecting to a powerful server via
XDMCP. The server used to run Slackware 11 and everything was working.
Recently I upgraded the server to Ubuntu Feisty and the client can no
longer use XDMCP (although it connects).

The client makes a successful XDMCP connection (I know this from
tcpdump) and starts talking with the server in X11. The login screen
comes up for about 2 seconds, but then something (an xorg error on the
client, I think) causes the client's X server to partially restart with
this passage in Xorg.0.log:
----------------------------------------
(II) Loading sub module "int10"
(II) LoadModule: "int10"
(II) Reloading /usr/X11R6/lib/modules/linux/libint10.a
(II) VESA(0): initializing int10
(II) VESA(0): Primary V_BIOS segment is: 0xc000
(II) VESA(0): VESA BIOS detected
(II) VESA(0): VbeVersion is 258, OemStringPtr is 0xc000260d,
        OemVendorNamePtr is 0x01040103, OemProductNamePtr is 0x01010100,
        OemProductRevPtr is 0x01110105
(II) VESA(0): VESA VBE Version 1.2
(II) VESA(0): VESA VBE Total Mem: 1024 kB
(II) VESA(0): VESA VBE OEM: Cirrus Logic GD-5436 VGA
(II) VESA(0): virtual address = 0x404b0000,
        physical address = 0xa0000, size = 65536
(==) VESA(0): Default visual is PseudoColor
(==) RandR enabled
----------------------------------------

The last message that I see in the tcpdump is the client sending:
----------------------------------------
Event: DestroyNotify, UnmapNotify, EnterNotify, DestroyNotify"
----------------------------------------
message to the server. 

After this the XDMCP session reinitializes successfully and the whole
cycle repeats indefinitely. 

Is there more logging I can turn on somewhere? What troubleshooting can
I do from here?

Thank you for your time.


Val