Your SVN repository might be located behind a proxy. Upsource doesn't have any UI to specify proxy settings. This article will tell you how to work around this issue.
Upsource uses SVNkit for communicating with an SVN repository. You will need to edit the "servers" file in the Subversion configuration and add the following lines in the "[global]" section:
http-proxy-host = proxy.example.com
http-proxy-port = 8080
http-proxy-username = myuserid
http-proxy-password = mypassword
Refer to SVNkit config guide for more details: http://svnkit.com/kb/user-guide-config-settings.html