WebDAV & Exchange 2003 - works in debug, fails without - vb.net

Question

So I have a very weird problem.
I am writing some code in VB.Net under .NET 2.0 which interfaces with MS Exchange 2003. Because of the Exchange 2003 "requirement" I am forced to write this code using WEBDAV.
The code itself is replicating, to some degree, a schedule management process. It's creating Appointments on the Exchange Server in response to inputs from the user and managing it's data internally in a SQL Server database.
The problem situation is this: A new person is assigned to be in charge of a meeting. The requirement says the program should generate a meeting cancellation request to the person removed from the meeting (if such a person existed) and a meeting request sent to the new person.
In the case of there being an existing person, what appears to happen is this:
The meeting cancellation request
gets sent
Exchange barfs and returns status code 500 (internal server error) during the set of
requests which send the meeting request to the new person.
However! While debugging this particular scenario, it works just fine for me, if I step through the code in the Visual Studio debugger. Left to it's own devices, it fails every time.
Just for yuk's sake, I added a Thread.Sleep(500) to the part after sending the cancellation request, and Exchange doesn't barf anymore...
So, my question!
If adding a Thread.Sleep to the code causes this error to go away, a race condition is implied, no? But, my code is running under a web application and is a totally single threaded process, from start to finish. The web requests I am sending are all in synchronous mode so this shouldn't be a problem.
What would I do next to try and track down the issue?
Try and divine if the race condition itself is in the .Net 2.0 BCL networking code?
Try and do some debugging on the Exchange server itself?
Ignore it, be glad the Thread.Sleep masks the problem and keep on going?
Any further suggestions would be wonderful.
In response to comment, I can post the failing function:
Private Shared Sub UpdateMeeting(ByVal folder As String, ByVal meetingId As String, ByVal oldAssignedId As String, ByVal newAssignedTo As String, ByVal transaction As DbTransaction)
If String.IsNullOrEmpty(meetingId) Then
Throw New Exception("Outlook ID for that date and time is empty.")
End If
Dim x As New Collections.Generic.List(Of String)
If oldAssignedId <> newAssignedTo AndAlso Not String.IsNullOrEmpty(oldAssignedId) Then
'send cancellation to old person
Dim lGetCounselorEmail1 As String = GetCounselorEmail(oldAssignedId, transaction)
Common.Exchange.SendCancellation(meetingId, New String() {lGetCounselorEmail1})
' Something very weird here. Running this code through the debugger works fine. Running without causes exchange to return 500 - Internal Server Error.
Threading.Thread.Sleep(500)
End If
x.Add(folder)
If Not String.IsNullOrEmpty(newAssignedTo) Then x.Add(GetCounselorEmail(newAssignedTo, transaction))
x.RemoveAll(AddressOf String.IsNullOrEmpty)
If x.Count > 0 Then
If Not Common.Exchange.UpdateMeetingAttendees(meetingId, x.ToArray()) Then
Throw New Exception("Failure during update of calendar")
End If
End If
End Sub
...but a lot of the implementation details are hidden here, as I wrote a set of classes to interface with Exchange WebDAV.

Answer 1

Ended up sticking with the Sleep and calling it a day.
My 'belief' is that I was erroneous in thinking that a WebRequest/WebResponse combo sent to Exchange through WebDav was an atomic operation.