OpenSAML user discussion

Hi there,
I have a little (hope so) problem with SAML.
After creating a response, I try to put my generated SAML into a byte-Array.
But I get such error-message:

java.lang.IllegalArgumentException: No attributes are implemented
        at
org.apache.crimson.jaxp.DocumentBuilderFactoryImpl.setAttribute(DocumentBuilderFactoryImpl.java:93)
        at org.opensaml.XML$ParserPool.get(XML.java:177)
        at org.opensaml.SAMLObject.toDOM(SAMLObject.java:158)
        at org.opensaml.SAMLSignedObject.toDOM(SAMLSignedObject.java:96)
        at org.opensaml.SAMLObject.toStream(SAMLObject.java:86)
        at org.opensaml.SAMLObject.toBase64(SAMLObject.java:110)
        at WAC_send.doSAML(WAC_send.java:101)
        at WACserver$1.run(WACserver.java:226)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:178)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:448)
        at
java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:197)
        at
java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
        at 
java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:144)
        at 
java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:136)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:99)

The same problem occurs if I try to put just the assertion to a stream and
so on.
What could that be?
It seems to miss some attributes but I don't set any and haven't set any
in the past...and it worked fine.

Greetings
Felix