ImportError: /usr/lib/python2.5/site-packages/lxml-2.1.2-py2.5-linux-i686.egg/lxml/etree.so: undefined symbol: xmlSchematronSetValidStructuredErrors
This line indicates that you installation of libxml2/libxslt/lxml is broken. You can test this independently of calibre with the following line:
python -c "from lxml import etree"
Until that command succeeds calibre will not work. I'm guessing this is caused by having som stale libraries onlyour system. You should search for libxml2 libxslt and libexslt and make sure that only the current versions are present.