I have the LDAP configuration configured for ProFTPD and I have modified the modules.conf file to include the LDAP module. However, every time I login with FileZilla I get 530 Login Incorrect. It does this for all users except those whose passwords are defined locally as well as in LDAP. The exact same setup works fine on my CentOS server and I've already tried re-installing it after purging the configuration files.
Do you have the appropriate /etc/pam.d file for proftpd to tell it to use LDAP?
The directory /usr/share/doc/libpam-ldap/examples/pam.d/ has examples. Normally you can just copy them all over to /etc/pam.d/
However on squeeze you have to change all occurances of "pam_unix_*" to "pam_unix" because all /lib/security/pam_unix_* files are gone and it now is just /lib/security/pam_unix.so. If you don't make these changes and log out you can't log back in, neither log in through the console or anything. You'll have to boot a rescue CD and change them that way.
So, for example in /usr/share/doc/libpam-ldap/examples/pam.d/ssh change:
The file for ftp (/usr/share/doc/libpam-ldap/examples/pam.d/ftp) looks like this: