I never saw that issues before. Since usually you got Proxy Authentication and Web Server Authentication both methods should not interefere each other. I assume a look at your configuration would be helpful. Maybe someone else has an idea, but I would go ahead and file an SR with support. If possible add a Feedback and a tcpdump which shows the problem.
Was notified that the machine was out of date with patch level. The techs brought it up to date and all works as designed.
Thanks for the time. Sorry for the confusion.