We are getting a message that there is a sign on failure even for the SYS account. However when I try to open up Sample data I can get in. An integrity check indicates that there is no error in the data.

How can you check the data integrity without signing on? A sign-on failure usually indicates an incorrect password.

End of previous day integrity check indicated no errors. We have this problem twice. Last Friday around mid morning, certain users could not access the system. However the system password could. There were only three users shown those with passwords all the other users had disappeared. I am able to access the sample data using the same password. What I am trying to find out is which file or configuration controls the user and group names.