LDAP Servers / Create New - Invalid Credentials

I' m trying to create an LDAP Server under User & Device-> Authentication on a FortiWiFi 60D v5.2.0, build 0589. The Active Directory server is Windows Server 2008 R2. When I click the icon by the Distinguished Name field it fills in the name. Using Server Port 389. I selected Bind Type = Regular. When I fill in the User DN and Password but I consistently get an Invalid credentials message. The LDAP Server is listed on the LDAP Servers page but when I click to Edit this and to Test the connection I again get the Invalid credentials message. This is the first time I' m trying to set this up. Can anyone tell what I' m doing wrong?