samba - Aug 2020 - Unable to do offline backup (start transaction stalls)

Hi,
I'm trying to do an offline backup but it gets stuck here:
root at gateway2:~ # samba-tool domain backup offline
--targetdir=/var/db/samba-backup
running backup on dirs: /var/db/samba4/private /var/db/samba4 /usr/local/etc
Starting transaction on /var/db/samba4/private/secrets
load: 0.06  cmd: tdbbackup 94560 [lockf] 316.69r 0.00u 0.00s 0% 2896k

An online backup completes OK:
root at gateway2:~ # samba-tool domain backup online --server=gateway2 -U darius
--targetdir=/tmp/samba
Password for [BEGER\darius]:
INFO 2020-08-30 16:50:47,106 pid:90384
/usr/local/lib/python3.7/site-packages/samba/join.py #1574: workgroup is BEGER
INFO 2020-08-30 16:50:47,106 pid:90384
/usr/local/lib/python3.7/site-packages/samba/join.py #1577: realm is
beger.com.au
<snip>
Processing section "[pcbackups]"
INFO 2020-08-30 16:50:52,799 pid:90384
/usr/local/lib/python3.7/site-packages/samba/netcmd/domain_backup.py #124:
Creating backup file
/tmp/samba/samba-backup-beger.com.au-2020-08-30T16-50-52.679372.tar.bz2...

I'd like to do an offline one so I can script it more easily.

This is FreeBSD 12.1-RELEASE-p7 with Samba 4.11.8 using ZFS.

Thanks.

--
Daniel O'Connor
"The nice thing about standards is that there
are so many of them to choose from."
 -- Andrew Tanenbaum