As of https://www.phpbb.com/community/viewtopic.php?f=46&t=2263586:

LDAP authentication does not support such scenario:

• LDAP server does not allow anonymous binding
• there is no special account for AD user
• but it allows binding for any registered user, but account suffix must be added to the user name

Here is my config example:

• LDAP server name: ldap://europe.company.net
• LDAP base dn: DC=company,DC=net
• LDAP uid: sAMAccountName
• LDAP e-mail attribute: mail

For binding I need such username: sAMAccountName@company.net, and I need that AD user is also: sAMAccountName@company.net with its password.

Apparently the modified file works for the user. With a few additional config parameters the LDAP authentication plugin would also support this scenario. However, I can't verify that (but I'm also used to LDAP suffixes here and there).