Logging in with local admin account after joining domain

Were you qualifying the username you entered with the local machine name? If not, it's likely Windows was assuming you were attempting to login as the administrator account for the domain instead of the local system.

In order to explicitly tell Windows that you want to login with the local administrator account you either need to qualify the username using the computer name like MYCOMPUTER\administrator or you can use the "dot notation" like this .\administrator. The dot is an alias that Windows recognizes as the local computer.

The only time the local account is "disabled" is when a server is promoted to a Domain Controller.