Internal Server Error / DirectAction

Hi all -

i'm working on an interface to a external system which is posting an  
xml file to our application. the external system http-post's the file  
to a url, with the filename as the single parameter in the url - eg:

http://claney.com/cgi-bin/WebObjects/Handler.woa/wa/DirectAction/ 
handler?filename=EBO_DETAIL_MM_35_0000000142.xml

My handler action simply writes the file to disk, and looks like this:

	public WOActionResults handoffAction() {
		String result = "";
		try {
			String content = request().contentString();
			String filename = request().formValueForKey("filename").toString();
			BufferedWriter out = new BufferedWriter(new FileWriter("/Library/ 
Handoffs/"+filename));
			out.write(content);
			out.close();
			result = "OK";
		} catch (Exception e) {
			System.out.println(e.getMessage());
			result = Utilities.getStackTrace(e);
			System.out.println(result);
		}
		Main pg = (Main)pageWithName("Main");
		pg.setResponseMessage(result);
		return pg;
	}


However, the external system is reporting an Internal Server error:

*****2006-04-05T09:07:26:5565 Info      NETWORK SERVICE@EVOLVIWEB03
*****
*****  3480 -  2773 Videcom.Utils.HttpHelper
Sending to http://claney.com/cgi-bin/WebObjects/Handler.woa/wa/ 
DirectAction/handoff?filename=EBO_DETAIL_MM_35_0000000142.xml
*****2006-04-05T09:07:27:1502 Error      NETWORK SERVICE@EVOLVIWEB03
*****
*****  3480 -  2773 Videcom.TravelPackOnline.KernelServer.Transport
Error when HTTPing file EBO_DETAIL_MM_35_0000000142.xml to http:// 
claney.com/cgi-bin/WebObjects/Handler.woa/wa/DirectAction/handoff.  
Exception = The remote server returned an error: (500) Internal  
Server Error


Unfortunately the external system is a supplier, and therefore I have  
no access to their code (and to be honest their developers are not  
being too helpful). there is nothing showing in the webobjects logs.

any ideas where i start to debug this ?

thanks, simon