After some research I have some new Information.
The Problem is not the WebService Client.
I have installed SoapUI and configured my WebService MOCK to use the following Library "xerces-1.2.3", which is older not newer (what I have thought first).
Then I tested the MOCK with SoapUI and figured out that the Error Message which I saw in my WebService Client was produced from the MOCK and not from the WebService Client.
I saw the Exception (org.xml.sax.SAXNotRecognizedException: Feature: http://xml.org/sax/properties/lexical-handler) in the Response-XML which the MOCK delivers to my Client.
So the MOCK Server is throwing this Exception but passes it in the Response-XML to the Client.
Now I know that the xerces-1.2.3 lib is not working with tomcat 6.0.33 and axis1.4
I think this is because xerces-1.2.3 implements SAX and not SAX2 which I think is in xercesImpl-2.x.x
In conclusion - Everything is fine now.