HTTP - Stream.Write Fails On Server But Works On Dev Machine?

problem with Stream.Write() where it works under a dev (Windows 7) environment but fails under a server environment (Windows 2008 R2 Enterprise)?

I'm basically working with some third party code which reads from a FileStream and writing to an Http request, nothing unusual and it's doing the following things:

Ensuring that Response.KeepAlive is false (was originally true but worked W7 and not W2K8 so tried false but it doesn't );
Ensuring that the Write includes the length of the bytes to write;
Ensuring that I Flush the stream afterwards;

The error that I encounter is the rather vague 'The request was aborted: The request was canceled.' with InnerException of 'Cannot close stream until all bytes are written.'. Now, my code works perfectly under Windows 7 so I still suspect that something is actively blocking this writing although I am not sure what, how and where.

One other possibility I wondered about was whether the encoding might be different under Windows Server 2008 R2 as opposed to Windows 7?

Here's a code snippet from the relevant method (a bit convoluted but, disclaimer, I didn't write it :-):

string boundary = string.Concat("-------------------------", DateTime.Now.Ticks.ToString("x", CultureInfo.InvariantCulture));
byte[] boundaryBytes = System.Text.Encoding.UTF8.GetBytes(string.Concat("
--", boundary, "
"));........