I have the same problem. Two files with sizes above 5MB throw cross-thread exception (just when first file upload is about to be finish)
Changing the buffer size does affect the issue, however if i put too large number into buffer size i get some other errors.
Błąd: Unhandled Error in Silverlight Application
Message: System.ArgumentNullException: Value cannot be null.
Nazwa parametru: key
w System.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument)
w System.Collections.Generic.Dictionary`2.FindEntry(TKey key)
w System.Collections.Generic.Dictionary`2.ContainsKey(TKey key)
w Telerik.Windows.Controls.RadUpload.OnFileUploadCompleted(Object sender, EventArgs e)
Now the buffer size is like 13107200B and I would like to keep it that way or even decrease it. But what if someone uploads a file that would exceed bs quota?