Certification failed for TO isolated action

[B]Dears,

from time to time my cluster shut it self down
i have no idea what is “Certification failed for TO isolated action”[/B]

2016-04-01 19:18:24 20379 [ERROR] WSREP: Certification failed for TO isolated action: source: fb18098c-f5d5-11e5-b23b-d6c2f78040de version: 3 local: 1 state: CERTIFYING flags: 65 conn_id: 9120130 trx_id: -1 seqnos (l: 2130783, g: 3969655, s: 3969649, d: -1, ts: 12265127177903945)
2016-04-01 19:18:24 20379 [Note] WSREP: /usr/sbin/mysqld: Terminated.

and here is the full log

2016-04-01 19:17:39 20379 [Note] WSREP: (fb18098c, ‘tcp://0.0.0.0:4567’) turning message relay requesting on, nonlive peers: tcp://33.33.33.33:4567
2016-04-01 19:17:41 20379 [Note] WSREP: (fb18098c, ‘tcp://0.0.0.0:4567’) reconnecting to 86935180 (tcp://45.79.179.6:4567), attempt 0

2016-04-01 19:18:24 20379 [Note] WSREP: evs::proto(fb18098c, GATHER, view_id(REG,37acae0e,72)) suspecting node: 86935180
2016-04-01 19:18:24 20379 [Note] WSREP: evs::proto(fb18098c, GATHER, view_id(REG,37acae0e,72)) suspected node without join message, declaring inactive
2016-04-01 19:18:24 20379 [Note] WSREP: declaring 37acae0e at tcp://11.11.11.11:4567 stable
2016-04-01 19:18:24 20379 [Note] WSREP: Node 37acae0e state prim
2016-04-01 19:18:24 20379 [Note] WSREP: view(view_id(PRIM,37acae0e,73) memb {
2016-04-01 19:18:24 20379 [Note] WSREP: save pc into disk
2016-04-01 19:18:24 20379 [Note] WSREP: forgetting 86935180 (tcp://33.33.33.33:4567)
2016-04-01 19:18:24 20379 [Note] WSREP: (fb18098c, ‘tcp://0.0.0.0:4567’) turning message relay requesting off
2016-04-01 19:18:24 20379 [Note] WSREP: New COMPONENT: primary = yes, bootstrap = no, my_idx = 1, memb_num = 2
2016-04-01 19:18:24 20379 [Note] WSREP: STATE EXCHANGE: Waiting for state UUID.
2016-04-01 19:18:24 20379 [Note] WSREP: STATE EXCHANGE: sent state msg: 47d61e65-f82e-11e5-a461-8a8519c6c4a4
2016-04-01 19:18:24 20379 [Note] WSREP: STATE EXCHANGE: got state msg: 47d61e65-f82e-11e5-a461-8a8519c6c4a4 from 0 (server1)
2016-04-01 19:18:24 20379 [Note] WSREP: STATE EXCHANGE: got state msg: 47d61e65-f82e-11e5-a461-8a8519c6c4a4 from 1 (server2)
2016-04-01 19:18:24 20379 [Note] WSREP: Quorum results:
2016-04-01 19:18:24 20379 [Note] WSREP: Flow-control interval: [91, 91]
2016-04-01 19:18:24 20379 [Note] WSREP: New cluster view: global state: 26f7d687-f400-11e5-a284-7a3269f0619f:3969651, view# 20: Primary, number of nodes: 2, my index: 1, protocol version 3
2016-04-01 19:18:24 20379 [Note] WSREP: wsrep_notify_cmd is not defined, skipping notification.
2016-04-01 19:18:24 20379 [Note] WSREP: REPL Protocols: 7 (3, 2)
2016-04-01 19:18:24 20379 [Note] WSREP: Service thread queue flushed.
2016-04-01 19:18:24 20379 [Note] WSREP: Assign initial position for certification: 3969651, protocol version: 3
2016-04-01 19:18:24 20379 [Note] WSREP: Service thread queue flushed.
2016-04-01 19:18:24 20379 [ERROR] WSREP: Certification failed for TO isolated action: source: fb18098c-f5d5-11e5-b23b-d6c2f78040de version: 3 local: 1 state: CERTIFYING flags: 65 conn_id: 9120119 trx_id: -1 seqnos (l: 2130781, g: 3969653, s: 3969620, d: -1, ts: 12265127145999324)
2016-04-01 19:18:24 20379 [Note] WSREP: Closing send monitor…
2016-04-01 19:18:24 20379 [Note] WSREP: Closed send monitor.
2016-04-01 19:18:24 20379 [Note] WSREP: gcomm: terminating thread
2016-04-01 19:18:24 20379 [Note] WSREP: gcomm: joining thread
2016-04-01 19:18:24 20379 [ERROR] WSREP: Certification failed for TO isolated action: source: fb18098c-f5d5-11e5-b23b-d6c2f78040de version: 3 local: 1 state: CERTIFYING flags: 65 conn_id: 9120130 trx_id: -1 seqnos (l: 2130783, g: 3969655, s: 3969649, d: -1, ts: 12265127177903945)
2016-04-01 19:18:24 20379 [Note] WSREP: /usr/sbin/mysqld: Terminated.

Also after installing Percona XtraDB i got a lot of access denied in logs

2016-04-01 19:18:09 20379 [Warning] Access denied for user ‘root’@‘localhost’ (using password: NO)
2016-04-01 19:18:12 20379 [Warning] Access denied for user ‘root’@‘localhost’ (using password: NO)
2016-04-01 19:18:15 20379 [Warning] Access denied for user ‘root’@‘localhost’ (using password: NO)

Waiting for any suggestions
Regards

We experienced exactly the same, running Percona XtraDB Cluster server 5.5.41 on EL (release 25.12.855.el6)