We have had issues installing all versions post 419 in our lab environment. Our lab is blocked off with firewalls and proxy servers. Using Wireshark and inspecting the packets, we found that v.419 were making calls to sourceforge. Versions 4.2.xx+ are now making calls to opencandy.winscp.net. We also see in our packets that versions 4.2.xx end up in Error Code 400 (Bad Requests).
The URL "opencandy.winscp.net" was also added to our proxy "whitelists" and non-cache lists to no avail.