Sudden pyexpat error with ElementTree

[Simon Pickles]

Hi,

I've been using ElementTree for a few weeks without problem, with 
Stackless Python.

Suddenly I have an error importing expat, in both application and console:

simon at simon-desktop:~$ python
Python 2.5.2a0 Stackless 3.1b3 060516 (release25-maint:60694M, Feb  9 
2008, 13:21:41)
[GCC 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
 >>> from xml.parsers import expat
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/site-packages/_xmlplus/parsers/expat.py", 
line 4, in <module>
    from pyexpat import *
ImportError: 
/usr/lib/python2.5/site-packages/_xmlplus/parsers/pyexpat.so: undefined 
symbol: PyUnicodeUCS4_Decode
 >>>

Google shows a few other sufferers, but reveals no answers. I have just 
rebuilt python to see if I had messed it up somehow, but the problem 
persists.

Thanks for your advice.

Simon