ASP.NET ActiveDirectoryMembershipProvider and port 445 (SMB)
While AD Membership is no longer recommended for use (use ADFS or something more modern instead), and is no longer being actively developed, we still see it from time-to-time here in support. One issue when using System.Web.Security.ActiveDirectoryMembershipProvider I've personally seen a couple times is the provider failing when port 445 is either inaccessible or blocked. When setting-up this provider with Active Directory, port 445 between the app server and Domain Controllers must be open and usable.