With an existing LDAP Server configuration set up through Instance Settings, the server's credentials (password) field is populated with an incorrect value when you go to edit said server. If you then save the configuration without re-entering the correct value, the wrong value will be stored.
1) configure your LDAP server with the correct values
2) save the settings
3) edit configuration again, do not change anything
4) save the settings again
5) edit configuration
6) click "Test LDAP Connection" --> connection to LDAP server fails
7) set the password again to the correct value
8) click "Test LDAP Connection" --> connection works
This cycle can be repeated with the same result.