Here's one to bend your noodle...I have AD installed on 5.1.4 and everything was working fine.  I wanted to see if I could remove the domain name from what was stored in the db so someone suggested checking the strip domain name in the Settings page of AD.

That didn't work, so I went to set everything back the way it was.  Problem is, if you log out, you can't log back in.  Except for me, and my test account.  I've had other people in the department go to the dnn site, and it logs them in no problem, automatically, which is what I want.  But if they hit log out and try to log back in, Hitting the Windows Login button, it won't log them in, and throws this in the Event Log:

AssemblyVersion: 5.1.4
PortalID: 0
PortalName: Cypress Web Portal
UserID: -1
UserName:
ActiveTabID: 39
ActiveTabName: Home
RawURL: /Login.aspx?ReturnUrl=%2fAdmin%2fExtensions.aspx
AbsoluteURL: /Default.aspx
AbsoluteURLReferrer: http://portal02/Login.aspx?ReturnUrl=%2fAdmin%2fExtensions.aspx
UserAgent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.2; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: 0848340f-e91c-4a92-99a1-5f954b6f6ec6
InnerException: Object reference not set to an instance of an object.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: DotNetNuke.Authentication.ActiveDirectory.ADSI.Utilities.AddADSIPath
StackTrace:
Message: System.NullReferenceException: Object reference not set to an instance of an object. at DotNetNuke.Authentication.ActiveDirectory.ADSI.Utilities.AddADSIPath(String Path, Path ADSIPath) at DotNetNuke.Authentication.ActiveDirectory.ADSI.Utilities.GetDomainByBIOSName(String Name) at DotNetNuke.Authentication.ActiveDirectory.ADSI.Utilities.GetUserEntryByName(String Name) at DotNetNuke.Authentication.ActiveDirectory.ADSI.ADSIProvider.GetUser(String LoggedOnUserName, String LoggedOnPassword)
Source:
Server Name: PORTAL02

this is all internal.  The biggest problem is, it was working FINE this morning.  And if No One could log in, that would be one thing, but I'm the only one it seems to like, I don't understand.  That and why the username is missing in this error message.

any advice?

Yeah, I changed it to visible = true on the settings.ascx page

The weird thing is, if you type in the domain in All caps, it seems to be okay, but only in some cases.  We're using two domains, one is a child of the other.  my personal id is in all small as is the domain, but everyone else seems to have to sign in using caps in the domain.  Thought it might be because it's the admin of the site, but there are two others, and they have to have all caps.

 I've started with another version - 4.9 of dnn because of some incompatibility issues with some other stuff, but I'm coming up against the same issue.

User goes to the page, and it autologs them in.  I'm starting with a new db, so I made the domain strip change before anyone logged in.  They're all being stored as their username only, no domain.  Logging out and logging in, using the Windows Login button to enter the username and pass doesn't work.  Still getting the same error message (no username):

AssemblyVersion: 04.09.05
PortalID: 0
PortalName: My Website
UserID: -1
UserName:
ActiveTabID: 38
ActiveTabName: Home
RawURL: /Home/tabid/38/ctl/Login/Default.aspx?returnurl=%2fHome%2ftabid%2f38%2fDefault.aspx
AbsoluteURL: /Default.aspx
AbsoluteURLReferrer: http://portal03.sacnrm.local/Home/tabid/38/ctl/Login/Default.aspx?returnurl=%2fHome%2ftabid%2f38%2fDefault.aspx
UserAgent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.2; Trident/4.0; GTB6; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.648; .NET CLR 3.5.21022; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: 38d23cbb-9060-49c8-8f98-ffc9b5dafa35
InnerException: Logon failure: unknown user name or bad password.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.DirectoryServices.DirectoryEntry.Bind
StackTrace:
Message: System.Runtime.InteropServices.COMException (0x8007052E): Logon failure: unknown user name or bad password. at System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail) at System.DirectoryServices.DirectoryEntry.Bind() at System.DirectoryServices.DirectoryEntry.get_NativeObject() at DotNetNuke.Authentication.ActiveDirectory.ADSI.ADSIProvider.IsAuthenticated(String Path, String UserName, String Password)
Source:
Server Name: PORTAL03