As I understand, your LDAP connection status shows Successful. However, unable to validate the user credentials even if it is right. Error > User credentials Invalid credentials.
Next Action Plan to identify the issue:
Follow the below link on how to collect logs using putty.
Putty 1 :
diagnose debug app fnbamd -1
diagnose debug enable
Follow the same steps to reproduce the issue: Go to >>Users and Authentication"->"Ldap Servers"-> select LDAP server-> click "Test Users Credentials
Use the below command to disable debug:
di de di
Simultaneously on Putty 2 :
diagnose sniffer packet any "host x.x.x.x" 6 0 l
Here x.x.x.x is the ldap server IP.
Use ctrl+C keys to stop the sniffer