Check the /etc/passwd file and the NIS or NIS+ passwd map on the local system to see if an entry exists for this user. If a user has simply forgotten the password, su(1M) and set a new one with the passwd(1) username command. This command automatically updates the NIS+ passwd map, but with NIS you will need to coordinate the update with the passwd map.
The Login incorrect problem can also occur with older versions of NIS when the user name has more than eight characters. If this is the case, edit the NIS password file, change the user name to have eight or fewer characters, and then remake the NIS passwd map.
If you cannot log in to the system as root, despite knowing the proper password, it is possible that the /etc/passwd file is corrupted. Try to log in as a regular user and su(1M) to root.
If that does not work, see the message su: No shell and follow most of the instructions given there. Instead of changing the default shell, make the password field blank in /etc/shadow.