When an SST runs using xtrabackup-v2 I see the following error in syslog on the joiner and the SST is unable to complete no matter how many times I restart mariadb:

-wsrep-sst-joiner: #007xbstream: Can't create/write to file './xtrabackup_logfile' (Errcode: 17 - File exists)

Whilst debugging this I eventually found there was still an .sst directory inside /var/lib/mysql that had a bunch of files in it, including xtrabackup_logfle. Once I deleted this directory hen the SST worked.

My suggested fix it to modify this file:

/usr/bin/wsrep_sst_xtrabackup-v2

and change this line:

mkdir -p ${DATA}/.sst

to:

rm -rf ${DATA}/.sst
mkdir ${DATA}/.sst

to ensure the .sst directory is always empty as that is what xtrabackup seems to expect. The .sst directory is in the list to be deleted for the default cpat options anyway, but that options does not clear the .sst directory soon enough to allow the sst to work.