Hello,
You may want to validate the bind account configured is able to login to your LDAP server.
On a Windows Server open ldp.exe
Click Connection | Connect and specify server name and port and click OK
Click Connection | Bind
Choose simple bind and enter your bind account name and passowrd and click Ok.
Some ldap servers may need to dn to login “cn=user,dc=domain,dc=com”
Others will take a simple username.
Use the same name format in BI that works with ldp.exe.
Also If you check the logs on your appliance you may get more details on the cause of the issue.
Errors relating to a directory sync should be found here on your U3 appliance.
C:\Program Files\BeyondTrust\BeyondInsight Gateway Services\DirectoryServices\logs