I have installed mongo v4.2.6 . I have created a users. sometime it gives authentication failed and after some time it resolved itself and now again same error and i am not getting any solution.
I have never observed this behaviour. I have often seen typing errors and lower vs upper case issues. At other time I have seen people running 2 instances at different times and mixing users from one instances to the other.
So it is most likely an user error rather than a server error.
More details might help to find out if it is really the case.