Brand new Windows 10 Install.
Can login with local user with no problem.
Can login with SOME Domain Accounts.
But others have strange problems with messages that read:
1) Can't sign into your account (which gives an option to SIGNOUT or DISREGARD).
2) If Disregard, then a notification appears that reads "You've been signed in with a temporary profile."
My domain account will login with no problem. During the first login we see all the "We're preparing Windows..." nonsense. Once finished... all is well.
The domain ADMINISTRATOR account behaves the same way.
But two of the user accounts that need to access this computer will not login properly.
But what is strange is that both of these (domain) accounts can login on other (upgraded) Windows 10 systems.
And both of these accounts have Microsoft LIVE accounts that function properly.
As I was writing this I solved it, but I thought I would post this anyway in case some other poor soul spends a week trying to debug it.
All of these accounts are managed on a Server 2008 system. They are all old... dating back 15 years. Probably starting on Server 2000. They've been migrated a few times.
When we originally set them up, we used roaming profiles. Somehow over the years the setting on the server for the profile location had been erased, but not for the other two problem accounts.
Up until Windows 10, this was not an issue. And for systems that had been upgraded from Windows 8, it was not a problem, presumably because the user profile was already established on the system. I speculate that because this was a NEW install, Windows 10 got confused when the domain user tried to log in. It would see in the login settings that there was a profile stored on the server. It was presumably a really old profile because we disabled roaming profiles years ago, but up to now leaving the setting caused no problems.
In the PROFILES tab for the USER on the SERVER, there is a setting for the profile path. Removing that solved my problem.
Hope this benefits someone else!