There is a debian package called ldap-utils which installs things like ldapsearch. This program lets you query the ldap server remotely via command line. This way, you can test if your server can be accessed from your owncloud-server (or if not because some firewall or something) and returns the correct information.
OwnCloud uses php5-ldap which also (like ldap-utils) depends on libldap-2.4-2, so I guess they use the same codebase.
If you want to stay graphical on windows, there is apache directory studio, a ldap browser which also lets you browse your ldap remotely, but I'm not sure which codebase it uses.
If you can query your ldap server and it returns the correct information, the problem must be in your "ldap user and group backend"-configuration.
(mostly) inactive, formerly running owncloud 5 - 5.0.12 on an Apache 2.2/PHP 5.3.3 using a PostgreSQL 8.4 on a Debian 6 (squeeze) server serving 11 users