Ran into this issue myself over the past few days on a fresh install of CentOS 7 connecting to a Windows Server 2008 R2 domain controller with active directory. I was able to use ldapsearch and the provided bin/auth tool to query the active directory server successfully but yet through the web interface for phabricator it kept rejecting with the error describe in the original post. After some debugging I determined it to be due to SELinux blocking the ldap connection from httpd. Turns out you can run the following command to allow for httpd to use php-ldap to connect out to the active directory server.