i could finally authenticate users in Alfresco against LDAP AD. But the syncronization (import groups and users) is still not working. Checking the log i get connection timeout errors.
17:14:20,922 Userystem WARN [security.sync.ChainingUserRegistrySynchronizer] Failed initial synchronize with user registries org.alfresco.error.AlfrescoRuntimeException: 06190000 User and group import failed
[Root exception is java.net.ConnectException: Connection timed out: connect]