I can recreate this in SoapUI 4.5.1 (Build [internal], Build Date 2012/06/27 13:56)
If I make a request that returns a large response then that large response is not shown (the old response remains on screen) and all subsequent responses also do not refresh the response window. However, the response size is being updated/reported and the large response that causes the problem is 34,713,409 bytes (yeh it's big!).
Looks liek a simple out-of-memory issue (as you might expect), so potentially the exception just needs to be handled more gracefully (originally I didn't realise the response wasn't updating).
Thu Nov 29 14:47:42 GMT 2012:ERROR:java.lang.OutOfMemoryError: Java heap space
java.lang.OutOfMemoryError: Java heap space
at java.util.Arrays.copyOf(Unknown Source)
at java.lang.AbstractStringBuilder.expandCapacity(Unknown Source)
at java.lang.AbstractStringBuilder.ensureCapacityInternal(Unknown Source)
at java.lang.AbstractStringBuilder.append(Unknown Source)
at java.lang.StringBuffer.append(Unknown Source)
at java.io.StringWriter.write(Unknown Source)
at org.apache.xmlbeans.impl.store.Cursor._save(Cursor.java:618)
at org.apache.xmlbeans.impl.store.Cursor.save(Cursor.java:2562)
at org.apache.xmlbeans.impl.values.XmlObjectBase.save(XmlObjectBase.java:212)
at com.eviware.soapui.support.xml.XmlUtils.serializePretty(XmlUtils.java:200)
at com.eviware.soapui.support.xml.XmlUtils.prettyPrintXml(XmlUtils.java:692)
at com.eviware.soapui.impl.wsdl.submit.transports.http.SinglePartHttpResponse.getContentAsString(SinglePartHttpResponse.java:98)
at com.eviware.soapui.impl.support.components.ResponseXmlDocument.propertyChange(ResponseXmlDocument.java:81)
at java.beans.PropertyChangeSupport.fire(Unknown Source)
at java.beans.PropertyChangeSupport.firePropertyChange(Unknown Source)
at java.beans.PropertyChangeSupport.firePropertyChange(Unknown Source)
at com.eviware.soapui.model.support.AbstractModelItem.notifyPropertyChanged(AbstractModelItem.java:85)
at com.eviware.soapui.impl.support.AbstractHttpRequest.setResponse(AbstractHttpRequest.java:716)
at com.eviware.soapui.impl.support.panels.AbstractHttpRequestDesktopPanel.afterSubmit(AbstractHttpRequestDesktopPanel.java:606)
at com.eviware.soapui.impl.wsdl.panels.request.AbstractWsdlRequestDesktopPanel.afterSubmit(AbstractWsdlRequestDesktopPanel.java:161)
at com.eviware.soapui.impl.wsdl.WsdlSubmit.run(WsdlSubmit.java:157)
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)