I have a similar issue with viewing the Dahua in "Web Service V3.0" but in my case, I'm able to view the Sub Stream, just not the Main Stream or the Preview (the one you ARE able to view). I'm guessing it may be a matter of streaming bandwidth? In your case, you can probably view the preview because the bandwidth it consumes is much lower than the Sub or Main stream — I'm just not certain what exactly is restricting it, the ISP? the camera? Let me know if you figuring anything out and I will do the same.