I have both DNN and AD authentication installed in my DNN instance.  However, for months now I have not enabled DNN authentication under Admin | Authentication, only AD was enabled with sync roles.  Having it setup this way works great, with the exception that I cannot login as the "HOST" account.  So, I checked both DNN and AD under authentication, and the "Hide Login controls" so as not to confuse my intranet user base.  I was able to login as host or my domain credentials, no problem.

Other domain users on the other hand were NOT able to login at all.  Some were, some were not.  Very strange.  Any reason why this happen?  I did try completely deleting one domain user's account on the portal and they still were not able to login at all.  After I turned of DNN authentication, they were able to login as normal, even having their account re-created on the portal and all AD groups added back.

If there is a way to fix this behavior, it would be great.  Then, I can login as either a DNN user, like HOST, or a domain user.

That's what I used to do, manually edit the Users table and make my AD account "IsSuperUser".  However, when I do this, and maybe you can see if you can duplicate this error, while I'm a SuperUser and try to change someone's security role and leave the box checked to "Notify" it throws an error and the email does not get sent.  When I remove myself from SuperUser and do the same thing, the notification works.  I was trying to avoid turning that on and off all the time and was not sure if having it on causes other "issues" with some functionality of the core.  Just a thought.