SoapFault: Unexpected end of file after null (sent as text/html)

I'm sending a message to a SoapUI mock and I get the following message back:

[2009.11.05 10:37:30:979] INFO: org.apache.cxf.interceptor.LoggingInInterceptor#logging - Inbound Message
----------------------------
ID: 37
Encoding: ISO-8859-1
Content-Type: text/html;charset=iso-8859-1
Headers: {content-type=[text/html;charset=iso-8859-1], Date=[Thu, 05 Nov 2009 09:37:30 GMT], Content-Length=[336], Serve
r=[Apache-Coyote/1.1]}
Payload:
 
   
      Server
      org.apache.xmlbeans.XmlException: error: Unexpected end of file after null
   
 


Restarting SoapUI usually solves the problem.
Any ideas, what's happening here?

And why is the content type of the message text/html and not xml?

Greetings,
  Malte