Node Joining Failure When wsrep_sst_method=mariabackup

Hello,

Simplest possible test environment for evaluating MariaDB clustering as follows (aka Steps to Reproduce):

Two fresh installs of CentOS 7 minimal, updated to all latest packages. Remove all firewall rules (non-internet facing test servers).

Imported MariaDB yum repo details.

yum install MariaDB-server MariaDB-backup perl-DBD-MySQL socat

As of this writing, MariaDB-server and MariaDB-backup vs 10.3.8 gets installed, all to default locations.

/etc/my.cnf.d.server.cnf on both machines include:

wsrep_on=ON
wsrep_provider=/usr/lib64/galera/libgalera_smm.so
wsrep_cluster_address=gcomm://10.0.0.1,10.0.0.2
wsrep_sst_method=mariabackup
wsrep_sst_auth=******:******

Bring up first MariaDB server with: galera_new_cluster

Create SST user on first MariaDB server using the same credentials specified above for wsrep_sst-auth:

GRANT RELOAD, LOCK TABLES, REPLICATION CLIENT, SUPER ON *.* TO ******@'localhost' identified by '******';

Bring up second MariaDB server with: systemctl start mariadb

File innobackup.backup.log on first MariaDB (the donor) includes lines such as:

Straming ./mysql/event.MYD to STDOUT
...done

Last line in innobackup.backup.log on that machine is: completed OK!

Second server still fails to join cluster. Log file on second MariaDB server ends with:

WSREP_SST: [INFO] Cleaning the binlog directory /var/log/mysql/bin as well (20180723 22:16:40.849)
removed ‘/var/log/mysql/bin/mariadb-master-bin.000001’
WSREP_SST: [INFO] Waiting for SST streaming to complete! (20180723 22:16:40.855)
2018-07-23 22:16:41 0 [Note] WSREP: (11d8586e, 'tcp://0.0.0.0:4567') turning message relay requesting off
2018-07-23 22:16:52 0 [Note] WSREP: 1.0 (mariadb1.local): State transfer to 0.0 (mariadb2.local) complete.
2018-07-23 22:16:52 0 [Note] WSREP: Member 1.0 (mariadb1.local) synced with group.
WSREP_SST: [INFO] Preparing the backup at /var/lib/mysql//.sst (20180723 22:16:52.240)
WSREP_SST: [INFO] Evaluating mariabackup --innobackupex   --apply-log $rebuildcmd ${DATA} &>${DATA}/innobackup.prepare.log (20180723 22:16:52.244)
rm: cannot remove ‘/var/lib/mysql//innobackup.prepare.log’: No such file or directory
rm: cannot remove ‘/var/lib/mysql//innobackup.move.log’: No such file or directory
WSREP_SST: [INFO] Moving the backup to /var/lib/mysql/ (20180723 22:16:52.644)
WSREP_SST: [INFO] Evaluating mariabackup --innobackupex       --move-back --force-non-empty-directories ${DATA} &>${DATA}/innobackup.move.log (20180723 22:16:52.647)
WSREP_SST: [ERROR] Cleanup after exit with status:1 (20180723 22:16:52.662)
2018-07-23 22:16:52 0 [ERROR] WSREP: Process completed with error: wsrep_sst_mariabackup --role 'joiner' --address '10.0.0.2' --datadir '/var/lib/mysql/'   --parent '5115' --binlog '/var/log/mysql/bin/mariadb-master-bin' : 1 (Operation not permitted)
2018-07-23 22:16:52 0 [ERROR] WSREP: Failed to read uuid:seqno and wsrep_gtid_domain_id from joiner script.
2018-07-23 22:16:52 0 [ERROR] WSREP: SST failed: 1 (Operation not permitted)
2018-07-23 22:16:52 0 [ERROR] Aborting

No [ERROR] lines on the first MariaDB server at all.

Have been running MariaDB clusters with rsync SST method for years just fine. Above example using rsync SST method instead works perfectly. Cannot get even this most simple test scenario to work with mariaback method. Grateful for any suggestions please.