Creating a DLP User with a password longer than 30 characters causes that user to be unable to log in with the intended password.
Release: 15.8 and 16.0
The following DLP User (ProtectUser) password pages limit the password length to 30 characters:
This issue is a bit deceptive because the user may think they are entering a password longer than 30 characters, but in actuality, the form is only accepting the first 30 characters, which is what is used for the stored password hash. The login page form does accept passwords up to 128 characters in length, so then when you attempt to log in as the user with the intended password, the resulting hash doesn't match the stored hash, so the login attempt fails.
DLP 16.0 RU1 will include updated forms to allow passwords up to 128 characters in length to match the maximum length supported for AD Login Users.