Critical Description
Prior to 5.0 the memcached admin_user was "_admin". In 5.0 it was changed to "@ns_server" and also other_users were added. But an upgraded doesn't change the admin_user.
http://src.couchbase.org/source/xref/spock/ns_server/src/ns_config_default.erl#267-268
On an upgrade admin_user should be changed. And other_users should be added.