Can't configure update mirror authentication

I'm running a migrated install if 5.0.119. I can get the update mirror to function correctly with no authentication, but once I turn on basic authentication and try to configure the credentials it stops working. The username field in the advanced configuration saves fine, but every time I send the Password field and save the configuration and return, I find that the field has been completely cleared out. I then tried NTLM authentication using the local admin account and a special "defs" user account on the system but neither would work. Has anyone got their mirror working with authentication?

I was hoping this version of the product would have an improved HTTP mirror that didn't spawn run-away threads that chewed up CPU cycles, but I guess I will have to go back to IIS.