We can not get Integrated Security to work

We are using Proget integrated with Active Directory. We are attempting to use integrated security but it isn't working. When we enable the "Enable integrated authentication", we are presented with a popup authentication for integrated security but regardless of if I type a valid or invalid user name/password, I get a 400 error and just get a blank screen. The only way to get back in is to go in the database and manually reset the setting back.

We are using the Web.UseIntegratedWebServer enabled. The Web.UserDirectoryId = 2 although that in itself is a little confusing since we have Built-In, LDAP and Active Directory with Multiple Domains. I believe 2 means to use Active Directory with Multiple Domains. Either way, we have configured the security appropriately for the users attempting to authenticate. Also, Web.LoadBalancingMode is also enabled although I'm not sure that is needed or relevant to this issue.

Product: ProGet
Version: 4.3.1

If you are getting a browser pop-up authentication (as opposed to a ProGet login screen), then it means Windows Integrated Authentication is not working. This is due to configuration of your server or domain.

Unfortunately, this is performed by IIS/Windows (not ProGet), which means the request is never reaching our software; this means we can't provide any information about why it's not working.

But this also means there are a lot of resources/articles you can find to give debugging on how to get it to work. I would suggest searches like, "Windows Integrated Authentication always authenticates" etc. Please post what you find!