One of my users can't login (one some machines).

In the shadow table/file/map there is a field that indicates how
long an account may be inactive before it is expired. On login,
the entry in /var/adm/lastlog, the inactive expire time and the
current date are compared. If the system determines that the user
is expired, he will get "Login incorrect", indiscernible from a
normal incorrect login. The fix is to change the user's shadow
entry.