5.6 MTR PXB failure in galera_sst_xtrabackup-v2-options on all nodes

Using the head of the PXB 2.3 trunk (commit: 1065aff88e48aa4f9abdec81658fe95e5595d65f)

$ xtrabackup --version
xtrabackup version 2.3.9 based on MySQL server 5.6.24 Linux (x86_64) (revision id: 1065aff)

(PXC builds its own PXB binaries)

This causes galera_sst_xtrabackup-v2-options to fail on all nodes.  The joiner fails with the following error message:

decrypt: incomplete data.
xbcrypt failed to decrypt

2017/08/26 03:35:33 socat[30555] E write(1, 0x265fbe0, 8192): Broken pipe

Using PXB 2.3.9 from the public repos works fine.