Invalid login password

Question

When I try to login in Ubuntu 16.04 with a valid password an error message is shown an screen saying invalid password, try again. I try to submit the password with no success, then I click on guest account after that I logged out and when I try to login again the same message is shown but the login was successful.

Can someone explain me how this why occurs and how to solve it?

Answer 1

I solved it by resetting /etc/shadow file permissions which were altered by root.