Main server mysql-error.log:

2021-01-22  2:00:50 0 [Note] WSREP: (37fc4f6e-87f8, 'tcp://0.0.0.0:4567') turning message relay requesting on, nonlive peers: tcp://192.168.5.4:4567
2021-01-22  2:00:51 0 [Note] WSREP: (37fc4f6e-87f8, 'tcp://0.0.0.0:4567') reconnecting to d42494c3-9d96 (tcp://192.168.5.4:4567), attempt 0
2021-01-22  2:00:54 0 [Note] WSREP: evs::proto(37fc4f6e-87f8, OPERATIONAL, view_id(REG,37fc4f6e-87f8,7)) suspecting node: d42494c3-9d96
2021-01-22  2:00:54 0 [Note] WSREP: evs::proto(37fc4f6e-87f8, OPERATIONAL, view_id(REG,37fc4f6e-87f8,7)) suspected node without join message, declaring inactive
2021-01-22  2:00:55 0 [Note] WSREP: (37fc4f6e-87f8, 'tcp://0.0.0.0:4567') reconnecting to 53ab8e76-bdc8 (tcp://192.168.5.3:4567), attempt 0
2021-01-22  2:00:58 0 [Note] WSREP: evs::proto(37fc4f6e-87f8, GATHER, view_id(REG,37fc4f6e-87f8,7)) suspecting node: 53ab8e76-bdc8
2021-01-22  2:00:58 0 [Note] WSREP: evs::proto(37fc4f6e-87f8, GATHER, view_id(REG,37fc4f6e-87f8,7)) suspected node without join message, declaring inactive
2021-01-22  2:00:58 0 [Note] WSREP: view(view_id(NON_PRIM,37fc4f6e-87f8,7) memb {
        37fc4f6e-87f8,0
} joined {
} left {
} partitioned {
        53ab8e76-bdc8,0
        d42494c3-9d96,0
})
2021-01-22  2:00:58 0 [Note] WSREP: New COMPONENT: primary = no, bootstrap = no, my_idx = 0, memb_num = 1
2021-01-22  2:00:58 0 [Note] WSREP: view(view_id(NON_PRIM,37fc4f6e-87f8,8) memb {
        37fc4f6e-87f8,0
} joined {
} left {
} partitioned {
        53ab8e76-bdc8,0
        d42494c3-9d96,0
})
2021-01-22  2:00:58 0 [Note] WSREP: Flow-control interval: [320, 400]
2021-01-22  2:00:58 0 [Note] WSREP: Received NON-PRIMARY.
2021-01-22  2:00:58 0 [Note] WSREP: Shifting SYNCED -> OPEN (TO: 553095643)
2021-01-22  2:00:58 12 [Note] WSREP: ================================================
View:
  id: 9f0915e2-dbf5-11e9-8a82-43733e3e049e:553095643
  status: non-primary
  protocol_version: 4
  capabilities: MULTI-MASTER, CERTIFICATION, PARALLEL_APPLYING, REPLAY, ISOLATION, PAUSE, CAUSAL_READ, INCREMENTAL_WS, UNORDERED, PREORDERED, STREAMING, NBO
  final: no
  own_index: 0
  members(1):
        0: 37fc4f6e-5c1b-11eb-87f8-73464d6f614e, frigga01
=================================================
2021-01-22  2:00:58 12 [Note] WSREP: Non-primary view
2021-01-22  2:00:58 12 [Note] WSREP: Server status change synced -> connected
2021-01-22  2:00:58 12 [Note] WSREP: wsrep_notify_cmd is not defined, skipping notification.
2021-01-22  2:00:58 0 [Note] WSREP: New COMPONENT: primary = no, bootstrap = no, my_idx = 0, memb_num = 1
2021-01-22  2:00:58 12 [Note] WSREP: wsrep_notify_cmd is not defined, skipping notification.
2021-01-22  2:00:58 0 [Note] WSREP: Flow-control interval: [320, 400]
2021-01-22  2:00:58 114648 [Warning] WSREP: gcs_caused() returned -1 (Operation not permitted)
2021-01-22  2:00:58 0 [Note] WSREP: Received NON-PRIMARY.
2021-01-22  2:00:58 12 [Note] WSREP: ================================================
View:
  id: 9f0915e2-dbf5-11e9-8a82-43733e3e049e:553095643
  status: non-primary
  protocol_version: 4
  capabilities: MULTI-MASTER, CERTIFICATION, PARALLEL_APPLYING, REPLAY, ISOLATION, PAUSE, CAUSAL_READ, INCREMENTAL_WS, UNORDERED, PREORDERED, STREAMING, NBO
  final: no
  own_index: 0
  members(1):
        0: 37fc4f6e-5c1b-11eb-87f8-73464d6f614e, frigga01
=================================================
2021-01-22  2:00:58 12 [Note] WSREP: Non-primary view
2021-01-22  2:00:58 12 [Note] WSREP: Server status change connected -> connected
2021-01-22  2:00:58 12 [Note] WSREP: wsrep_notify_cmd is not defined, skipping notification.
2021-01-22  2:00:58 12 [Note] WSREP: wsrep_notify_cmd is not defined, skipping notification.
2021-01-22  2:00:58 114051 [Warning] WSREP: Send action {(nil), 1880, WRITESET} returned -107 (Transport endpoint is not connected)
2021-01-22  2:00:58 114659 [Warning] WSREP: Send action {(nil), 1880, WRITESET} returned -107 (Transport endpoint is not connected)
2021-01-22  2:00:58 113489 [Warning] WSREP: Send action {(nil), 1864, WRITESET} returned -107 (Transport endpoint is not connected)
2021-01-22  2:00:58 113398 [Warning] WSREP: Send action {(nil), 1864, WRITESET} returned -107 (Transport endpoint is not connected)
2021-01-22  2:00:58 114661 [Warning] WSREP: Send action {(nil), 1864, WRITESET} returned -107 (Transport endpoint is not connected)
2021-01-22  2:00:58 114052 [Warning] WSREP: Send action {(nil), 720, WRITESET} returned -107 (Transport endpoint is not connected)
2021-01-22  2:00:58 113829 [Warning] WSREP: Send action {(nil), 696, WRITESET} returned -107 (Transport endpoint is not connected)
2021-01-22  2:00:58 114576 [Warning] WSREP: gcs_caused() returned -107 (Transport endpoint is not connected)
2021-01-22  2:00:58 113491 [Warning] WSREP: Send action {(nil), 1864, WRITESET} returned -107 (Transport endpoint is not connected)
2021-01-22  2:00:58 114571 [Warning] WSREP: Send action {(nil), 140080358372240, WRITESET} returned -107 (Transport endpoint is not connected)
2021-01-22  2:00:58 114664 [Warning] WSREP: Send action {(nil), 38654734216, WRITESET} returned -107 (Transport endpoint is not connected)
2021-01-22  2:00:58 113490 [Warning] WSREP: Send action {(nil), 704, WRITESET} returned -107 (Transport endpoint is not connected)
2021-01-22  2:00:58 113084 [Warning] WSREP: Send action {(nil), 1184, WRITESET} returned -107 (Transport endpoint is not connected)
2021-01-22  2:00:58 114656 [Warning] WSREP: Send action {(nil), 696, WRITESET} returned -107 (Transport endpoint is not connected)
2021-01-22  2:01:36 0 [Note] WSREP: (37fc4f6e-87f8, 'tcp://0.0.0.0:4567') reconnecting to d42494c3-9d96 (tcp://192.168.5.4:4567), attempt 30
2021-01-22  2:01:40 0 [Note] WSREP: (37fc4f6e-87f8, 'tcp://0.0.0.0:4567') reconnecting to 53ab8e76-bdc8 (tcp://192.168.5.3:4567), attempt 30
2021-01-22  2:02:21 0 [Note] WSREP: (37fc4f6e-87f8, 'tcp://0.0.0.0:4567') reconnecting to d42494c3-9d96 (tcp://192.168.5.4:4567), attempt 60
2021-01-22  2:02:25 0 [Note] WSREP: (37fc4f6e-87f8, 'tcp://0.0.0.0:4567') reconnecting to 53ab8e76-bdc8 (tcp://192.168.5.3:4567), attempt 60
2021-01-22  2:03:06 0 [Note] WSREP: (37fc4f6e-87f8, 'tcp://0.0.0.0:4567') reconnecting to d42494c3-9d96 (tcp://192.168.5.4:4567), attempt 90
2021-01-22  2:03:10 0 [Note] WSREP: (37fc4f6e-87f8, 'tcp://0.0.0.0:4567') reconnecting to 53ab8e76-bdc8 (tcp://192.168.5.3:4567), attempt 90

...and so on, repeats until mariadb is killed on this host.