[Zope] java crashes zope

I have a small applet compiled under sun java which works when tested on local disk. When trying to run it after uploding it to zope, java asks for reconfirmation of username and password and then crashes zope and sometimes my client pc. If I restart zope (with webmin) and reconnect to the site I am again asked for user and password by java but this time the applet runs perfectly. Is this a bug in this version of java or is there another reason???? Any thoughts appreciated