No, your previous assumption is correct and password file does not have to be there. Go to /etc directory and look for passwd and group files... passwd file contains the user database, with the fields giving the
username, real name, home directory, encrypted password, and other information about each user. Group file is just like passwd file but describes groups instead of users. May be there are some inconsistencies there. I would also look at differencies in network configuration between your previous install and current because if there are some networking parameters that require thsi authentication, oracle may look for passwrod file by default.