Hi Christian,
you could use the command “ldapsearch” from the package “ldap-utils” to check the connection from command line:
ldapsearch -D <user> -h <ldap_server> -w <password> -x -b <base_DN_for_search>
My guess would be: The firewall is blocking the access to the LDAP server/port. You could check this with a “telnet” or “netcat” command.
Best regards
Lars