login on server suddenly not working

I have just set up a 10.5.1 server a couple of weeks ago. I have set up an admin account and a open directory admin account. Suddenly today, I can't log in with the admin account (by loggin in I mean on the actual server, not connecting to it) - I can only log in with the open directory account. When logged in as the open directory user, I have tried to check if the user is still valid, and it works in the sense that I can perform administrative tasks with it (change system prefs, change in the server admin etc.) but I still can't log in on the account (meaning to the home folder - which I have checked, is still there). Any ideas? TIA, Peter