Some firewalls blacklist HTTP/HTTPS requests when the request does not include a user-agent string. When SourceTree attempts to log into github (eg Clone/New -> browse hosted projects), the requests to api.github.com:443 fail. The full header is below.

CONNECT api.github.com:443 HTTP/1.1
Proxy-Authorization: Negotiate [redacted]
Host: api.github.com
Proxy-Connection: Keep-Alive

Additionally, our corporate firewall gives a 200 OK response with an error page indicating that user-agent filtering is blocking this request, but sourcetree does not handle this response in a meaningful way; instead, it repeats the request (which fails) and alerts a Login error dialog box with the message:

"Login failed for your GitHub account.

The remote service said: 'The underlying connection was closed: An unexpected error occurred on a send.'"