IllegalStateException in EncodingSerializer

[Andreas Hartmann]

Hi Cocoon devs,
I get an IllegalStateException in the console when using
the components.serializers.XHTMLSerializer:
java.lang.IllegalStateException
        at 
org.apache.cocoon.components.serializers.EncodingSerializer.recycle(EncodingSerializer.java:123)
        at 
org.apache.cocoon.components.serializers.XMLSerializer.recycle(XMLSerializer.java:109)

The EncodingSerializer source:
<snip>
122     public void recycle() {
123        if (processing) throw new IllegalStateException();
</snip>
It happens only in a certain application, apart from that it is
working fine. Any hints where I could start searching the cause?
Thanks in advance,
-- Andreas