Traceroute can help you see if there is a choking point where traffic is no longer forwarded. Like at the point where it's going from your ISP's network to whichever network comes next before it hits SE's servers.

Or just try with WTFast and see if it works that way. They have a free trial so you can test and see if it works at all.

Other than that, with some anti-malware and other security software it's not enough to turn them off, but have to be uninstalled completely to get rid of any interference.

If you still have problems and want help from other players, list more information, such as which ISP you're on, what anti-malware and other security software you have installed, other information about your computer...
Or try your luck with SE support.