ClickOnce app. installation failure

We have a clickonce application deployed to a webserver. The virtual
directory has SSL enabled. When trying to download and install the app on a
client machine on a client's site the download fails with the following error:
-----
PLATFORM VERSION INFO
Windows : 5.1.2600.131072 (Win32NT)
Common Language Runtime : 2.0.50727.42
System.Deployment.dll : 2.0.50727.42 (RTM.050727-4200)
mscorwks.dll : 2.0.50727.42 (RTM.050727-4200)
dfdll.dll : 2.0.50727.42 (RTM.050727-4200)
dfshim.dll : 2.0.50727.42 (RTM.050727-4200)
SOURCES
Deployment url : https://<server>/<virtual-dir>/<application>.application
ERROR SUMMARY
Below is a summary of the errors, details of these errors are listed later
in the log.
* Activation of https://<server>/<virtual-dir>/<application>.application
resulted in exception. Following failure messages were detected:
+ Object reference not set to an instance of an object.
COMPONENT STORE TRANSACTION FAILURE SUMMARY
No transaction error was detected.
WARNINGS
There were no warnings during this operation.
OPERATION PROGRESS STATUS
* [15.04.2008 08:26:13] : Activation of
https://<server>/<virtual-dir>/<application>.application has started.
ERROR DETAILS
Following errors were detected during this operation.
* [15.04.2008 08:26:13] System.NullReferenceException
- Object reference not set to an instance of an object.
- Source: System
- Stack trace:
at System.Net.WebRequest.get_PrefixList()
at System.Net.WebRequest.Create(Uri requestUri, Boolean useUriBase)
at System.Net.WebRequest.Create(Uri requestUri)
at
System.Deployment.Application.SystemNetDownloader.DownloadSingleFile(DownloadQueueItem next)
at System.Deployment.Application.SystemNetDownloader.DownloadAllFiles()
at
System.Deployment.Application.FileDownloader.Download(SubscriptionState
subState)
at
System.Deployment.Application.DownloadManager.DownloadManifestAsRawFile(Uri&
sourceUri, String targetPath, IDownloadNotification notification,
DownloadOptions options, ServerInformation& serverInformation)
at
System.Deployment.Application.DownloadManager.DownloadDeploymentManifestDirectBypass(SubscriptionStore
subStore, Uri& sourceUri, TempFile& tempFile, SubscriptionState& subState,
IDownloadNotification notification, DownloadOptions options,
ServerInformation& serverInformation)
at
System.Deployment.Application.DownloadManager.DownloadDeploymentManifestBypass(SubscriptionStore
subStore, Uri& sourceUri, TempFile& tempFile, SubscriptionState& subState,
IDownloadNotification notification, DownloadOptions options)
at
System.Deployment.Application.ApplicationActivator.PerformDeploymentActivation(Uri activationUri, Boolean isShortcut)
at
System.Deployment.Application.ApplicationActivator.ActivateDeploymentWorker(Object state)
COMPONENT STORE TRANSACTION DETAILS
No transaction information is available.
-----

First I should say that the app. can be installed fine on my machine, so I
don't think it's a server configuration problem.

The client machine is behind a proxy server and is running on an OS running
in a different language to ours (German vs. our UK). I have searched for
anything I can think of regarding the "PrefixList" property that appears as
the last part of the stack trace of the WebRequest class but cannot find any
likely sources of a NullReferenceException.

Could this be caused by the proxy server preventing the .deploy files being
downloaded? I tried deleting one of the deploy files, but that gives a
different error stating that the file couldn't be found.

Can anyone out there shed some light on what might be going on or things
that can be tried to discover the exact cause?

Thanks in advance
.