Authenticate with AD on a LTM VIP

I would also look at the traffic between the LTM and DC. If it's pure LDAP, then you should be able to insert a WireShark capture right there and see what the LDAP looks like. If you don't see any traffic at all, or it fails to connect before any LDAP data passes, then you may have a layer 4 issue somewhere. If, however, you see LDAP data going back and forth, then you should also see any failures coming from the DC.