Have you allowed remote admin?
This happened to me just yesterday.
Here's what worked for me to solve the problem… perhaps this will work for you as well…
This is going to sound foolish or rude at first, but please bear with me… I'll explain.
When the Admin Console window first appears and you log in, type in the user name and then type in the password. The "OK" button should then be enabled while you are typing the first few characters of the password.
It's my habit to copy a password from my keychain and then paste it into the password field, and then do the same for the username. Some password systems do not work when I do this (for legimate reasons), and the Server Admin Console log-in is one of those systems. After I tried the copy and paste method, the login window wouldn't enable the "OK" button no matter what I did. I had to cancel out of that window and start from the beginning.
I presume that this kind of thing is done to prevent crackers from rapidly trying to log in with a list of candidate passwords. Though that practice might be bit annoying, I think it's prudent.
I don't know that this will solve your problem, but I hope it will.