The problem is not at LDAP authenticator level but more generally in XWiki which does not support "." in documents name.
As it will be too difficult to correct soon on XWiki and it's really needed feature, I propose to add a workaround in LDAP : replace "." with something else like "_" or nothing when creating/updating the XWiki user from LDAP. The only problem here is that login and user name will be different but it's not big issue in my point of view.