I’ve been having the same problem with version 10.x and now 11. I have my proxy settings configured correctly in Yast but when I test it I get back:
Proxy return code: HTTP/1.1 407 Proxy Authentication Required. I have proper credentials configured but it doesn’t seem to pass them to the proxy server. Because of this, updating fails. However, Firefox will connect through my proxy server without any problems using the same credentials.