Qt program crashes on Feisty Fawn due to X server

I just received a new Dell Inspiron 9400 PC with a ATI Technologies
Inc Radeon Mobility X1400 graphics card. I installed Kubuntu as I had
on my previous PC (Dell Inspiron 9300) and recompiled our program.

Now I get the errors:

W: X Error: BadRequest (invalid request code or no such operation) 1
  Extension: 219 (Uknown extension)
  Minor opcode: 0 (Unknown request)
  Resource id: 0x1
W: X Error: BadRequest (invalid request code or no such operation) 1
  Extension: 210 (Uknown extension)
  Minor opcode: 0 (Unknown request)
  Resource id: 0x1
W: X Error: BadRequest (invalid request code or no such operation) 1
  Major opcode: 125 ()
  Resource id: 0x1
W: X Error: BadLength (poly request too large or internal Xlib length
error) 16
  Major opcode: 119 (X_GetModifierMapping)
  Resource id: 0x1

I tried several thinsg, such as a reconfigure of the xserver-xorg, but
in vain. After googling around and trying to run the program, I have a
suspiscion that it is a threading problem. It only occurs (so I
think) when I enter or leave a renderthread that creates a pixmap
for the plot we make.

I also noticed that similar warnings are generated by a Fedora Core 5 system,
but without the program crashing.