What is your monitor accomplishing for the 24389 port? Assuming we have access to the port, I'd run wireshark on the LDAP server and SSL decrypt the traffic between LTM and the LDAP server.
https://support.f5.com/kb/en-us/solutions/public/9000/300/sol9311.html provides a basic overview of our LDAP monitors but at the bottom of the SOL are further references for LDAP/S monitors and potential issues when using TLS. The Wireshark capture will show you the bind success/failure if it even gets that far. From there you should be able to see what's up.