These errors are due to the fact that no XSL Transformer is available in Jakarta Ant 1.5 per default. When you run on JDK 1.4 you have all that for free.
Solution: the xmlApis.jar from Ant 1.5Beta1 do not contain javax.xml.transform Copy one that contains these classes to $ANT_HOME/lib. The one from Xalan 2.3.1 is just fine: xml-apis.jar. It is in xdoclet/lib for your convenience.
javax.xml.transform.TransformerFactoryConfigurationError: Provider for javax.xml.transform.TransformerFactory cannot be found
Solution: Copy xalan.jar to $ANT_HOME/lib