Not an issue anymore. This was the "fix" to maybe help understand & diagnose...
One the system administrators page I noticed that one of the other admins I had setup somewhat recently had a "created date" from many years ago. Now, to be fair, I can't recall if I had actually renamed a previous account or created a new one when I recently updated that other user. It also showed that the other user had never logged in, and the other user was also not even able to login.
So my "fix" for the other user was to delete that account (this was after several attempts to change that password) and then re-create it. Once that happened, the other user's account worked fine again.
And then once that happened, I was able to create new users again including the one I was attempting to do when I started this thread.
In summary - it appears that if somehow one of the system admin accounts gets corrupted that it then prevents other system admins from being created.