Interesting bug - it would be useful to report it to Nokia.
Anyway, you can solve it by either saving the file to isolated storage first, or copying it to MemoryStream locally:
HttpClient c = new HttpClient();
Stream orgImageStream = await c.GetStreamAsync(imageUri);
MemoryStream ms = new MemoryStream();
await orgImageStream.CopyToAsync(ms);
ms.Seek(0, SeekOrigin.Begin);
var imageSource = new StreamImageSource(ms);
But don't forget to use all Streams in using (Stream ... ) { ... } to Dispose them properly!