If the "Check Names Filter" is left blank, searching via LDAP will result in "Bad Search Filter".
According to this http://www.personal.psu.edu/sal21/blogs/stuff/2009/03/creating-a-custom-ldap-filter.html, the default search filter should be
(&(mail=*)(|(mail=%s*)(cn=%s*)(sn=%s*)(givenName=%s*)))
