Thanks for PM. I tested it and could reproduce the issue. The problem is that the proxy does require authentication – but it accepts any user name. I tried an empty name and it worked.
If you are using this proxy as the default connection, just check the "Proxy Authentication" box and leave the login fields empty. If you are using a proxy list, use a line like this (where "user" can be anything, for example just "user"):
$SOCKS$user@123.123.123.123:1080