Have you tested using a different browser or browser/OS combination to verify that it is indeed limited to just that browser/OS combination? Have you checked to see if you’re running a browser extension that blocks any tracking code or cookies? Some of these extensions will also block items from social networks, so it’s worth disabling those temporarily to see if that resolves the issue. Since the Twitter timeline widget also appears to be working in all browsers in Mac OS, I would bet that there’s something in your specific browser blocking Twitter connections as the Timeline widget is loaded from an iframe from Twitter directly.
I checked our Publicize logs on our end and everything appears to be working properly. I did notice that you have a plugin called “clone-posts” installed. Publicize only works with newly published posts, so you may run into issues if you use a plugin to clone or copy posts.
Let me know if you have any questions.