Internal MariaDB error code: 1927 ('Connection was killed')

Edit: filed a bug report at https://jira.mariadb.org/browse/MDEV-14493 Mariadb nodes freeze occasionally with the following error in the logs, any clues? (drupal app, db freezes on clearing application cache) [Warning] WSREP: Failed to apply app buffer: seqno: 903792, status: 1 Nov 23 21:42:55 websrv4 mysqld: #011 at galera/src/trx_handle.cpp:apply():351 Nov 23 21:42:55 websrv4 mysqld: Retrying 2th time Nov 23 21:42:55 websrv4 mysqld: 2017-11-23 21:42:55 140081879742208 [Warning] WSREP: BF applier failed to open_and_lock_tables: 1927, fatal: 0 wsrep = (exec_mode: 1 conflict_state: 5 seqno: 903792) Nov 23 21:42:55 websrv4 mysqld: 2017-11-23 21:42:55 140081879742208 [ERROR] Slave SQL: Error executing row event: 'Connection was killed', Internal MariaDB error code: 1927