Since 13.10.3 beta Dopus has been notifying me as usual that there is an update.
When I click to download it starts, the progress bar gets to a certain point, waits for several seconds, the clears and the update box returns back to 'the update is ready to be downloaded' message.
Downloading and subsequently updating works fine manually via the website.
I'm not seeing any Windows Defender/Firewall messages, and there's nothing wrong with my internet connection.
Not sure if this is just something weird about my PC or an issue at Dopus Mansions.
It could be a regional issue with the CDN (although the web download would usually use the same thing). Which part of the world are you in?
Defender can also block things without saying anything, in my experience. And these days it is one the of the worst antivirus out there in terms of causing problems for no valid reason. It may not be involved, but I would not rule it out without checking.