Hi,
I also experienced the domain login refusal problems, in the past with Redhat 8 and recently with SME 6.5b2. Actually it appears to be quite simple to sort if you have a clearer understanding of the mechanism involved - I think.
The principal of domain login is that the accounts are not setup on the workstation but on the server (domain controller - DC). This is for centralised management of accounts. When you first attach a workstation to the domain controller/server it needs the initial login to be that of the server administrator, i.e. "admin" in the case of SME. This generates a secure link or relationship between that workstation hardware and the server. Once this is done you normally reboot the workstation and then use any of the already setup server user accounts to login to the server.
Note: the server admin login from the workstation only happens once at the initial setup of the domain attachment. Note also that you do not setup user accounts on the workstation for the domain - only on the domain controller server.
I hope this helps.
Blue.