JNLPException: BindException: Address already in use

hello Gregg, hello all,
a customer is getting the following exception when he tries to start my application using JavaWebStart. I don't know yet whether this happens all the time, or only once in a while. (I suppose the latter.)
I searched through the net but it seems that noone else had these problems with JavaWebStart, or at least that the causing exception (BindException) is not that strictly related to JavaWebStart and rather a consequence of the net/proxy/firewall settings.
Whatever the reason is - I can't reproduce this error, thus I can't really tell my customer why this happens and how to avoid it.
Any ideas?

JNLPException[category: Download-Fehler : Exception:
java.net.BindException: Address already in use: connect : LaunchDesc:
null ] at com.sun.javaws.cache.DownloadProtocol.doDownload(Unknown
Source) at
com.sun.javaws.cache.DownloadProtocol.getResource(Unknown Source)
at com.sun.javaws.LaunchDownload.downloadJarFiles(Unknown Source)
at com.sun.javaws.LaunchDownload.downloadEagerorAll(Unknown Source)
at com.sun.javaws.Launcher.downloadResources(Unknown Source) at
com.sun.javaws.Launcher.handleApplicationDesc(Unknown Source) at
com.sun.javaws.Launcher.handleLaunchFile(Unknown Source) at
com.sun.javaws.Launcher.run(Unknown Source) at
java.lang.Thread.run(Thread.java:536)

Thank you,
Chantal