Hi Andreas,
I do not know what is causing your problem, and whether your problem is solved in ant 1.6.1.
Maybe it is a problem connected to Eclipse integration.
I do not really have a clue why you are getting this illegal argument exception.
I was hit by that a few days ago on a different project, and it's an endorsed problem, due to a wrong Xerces version. Now, don't ask me how to fix that in eclipse, but if you can make ant use Cocoon's xerces you should be fine.
Ciao,
-- Gianugo Rabellino Pro-netics s.r.l. - http://www.pro-netics.com Orixo, the XML business alliance - http://www.orixo.com (Blogging at: http://www.rabellino.it/blog/)